Tim,
Thanks for the detailed explanation and helping me to narrow down my decision points. I respect the way to patiently respond on questions with authoritative details over the subject.
(1) I am in need of video cam but I guess that may not be a criteria since this video cam of 70D will not have the functionality that the typical camcorder offers. Also the storage will also be a constraint. Therefore, I will consider this facility only as an added advantage and not a prime reason.
Consumer camcorders track focus exceptionally well, but they aren't as good in the lens department. The advantage of using a DSLR as a video camera is that you can swap lenses, find something with a nice low focal ratio for a tack sharp subject and softly de-focused background, etc. But typically these cameras don't track focus (at least not very well.) This has to do with how the focus system works when they are in video mode (the camera's best focusing system using sensors on the floor of the camera... not on the sensor itself. This focus system can only be used when the reflex mirror is in the "down" position so it can bounce light into the sensors. When in video mode, the mirror has to be in the "up" position, hence there's no way to use the better and faster focus system.
The 70D new feature is a dual-pixel AF system on-the-sensor. It's able to do "phase detect" auto-focus (just like the good focus sensors on the floor of the camera) but it can do this is EVEN in live view or video modes (a typically DSLR cannot). The focus system is much faster and due to the way "phase detect" focus works, not only does a camera know when something is in focus vs. out of focus... it actually knows WHICH direction to adjust focus (closer in vs. farther away) to correct for focus and it even knows how far. This system tends to not "hunt" for focus ... it snaps right to it.
There is one downside... apparently there are rules on trade and how products are classified. There's a tariff that gets applied to video cameras that does NOT get applied to still cameras. Then still cameras started doing video and the lines got blurry. So apparently the rules are... as long as the camera cannot record more than 30 minutes of continuous video, it doesn't have to get classified as a video camera and the tariffs don't have to be applied. It's an arbitrary rule and a bit frustrating, but the camera manufacturer didn't come up with the rule and ALL DSLR CAMERAS have the same restriction because it's not a technology problem.... it's a legal problem.
(2) I have read you other posts also on the cross type focusing point of many on 70D vs 1 on 6d. Though, I have not understood the 'practical' implications of it. For example, if I am taking a snap of a flying bird or of my family what way different camera will behave or I will have to behave while using each one of them.
That's a good question. When the camera focuses, it typically uses that "phase detect" auto-focus system that I mentioned above. The system sends light through a tiny prism (beam splitter -- but it's a prism). Since a prism has an axis (you can stand a prism upright, or lay it on it's side, etc.) it's going to split light in a direction orthogonal to the prism.
To help you visualize what's happening... suppose you print a photograph, take a pair of scissors and cut the photo into two halves. Now place the two halves next to each other. Slide them up or down along the cut and you can imagine that one point the pattern in the photo align for the two halves (they "match up") but at all other points they don't match up and it's obvious that they've been cut into two halves. More to the point... at a glance your eye can tell if the right half is too high or too low compared to the left half. Your eye can even tell how far these two halves are mis-aligned. This is a bit like how the phase-detect auto-focus system works.
While this works for most subjects... subjects that have a pattern and direction to them can be a problem -- especially if you keep in mind that the auto-focus sensor cannot see the whole image... it can only see a tiny area of it.
Suppose you are photographing a picture of a white picket fence. If the photo were cut through the middle horizontally, then it would be really obvious that the pickets in the top half do not line up with the pickets in the bottom half. BUT... if you cut the photo vertically instead of horizontally... NOW when the two halves are mis-aligned... you can't quite tell anymore. Because the pattern you were using (looking at the white picket fence) is parallel to the direction of the cut. Sure... if you look at the "whole" image you can probably tell. But the sensor can only see a tiny part of it. If the mis-alignment doesn't show up in the little tiny bit of the photo that the sensor can see, then the sensor can be fooled into think it has focused the shot... even if it is not focused.
So really... the question is... is your focus sensor aligned parallel to the direction of your pattern... or does it run orthogonal to the direction of your pattern.
When the pattern is intricate and runs in many direction then the camera has no problem. It's only a problem when the pattern takes on a stripe-like nature... tall grasses, fences, etc. Usually some of the sensors are oriented with one axis, while others have the opposing axis.
Enter the "cross type" point. The cross type point has a prism which splits the light into two orthogonal axes ... at the SAME TIME! Now when you take the photo of the picket fence, the focus system cannot be fooled... because while it couldn't find the mis-alignment in one axis, it was obvious when it checked the other axis. Hence... a harder-to-fool focus system and more accurate focus in your shots. They also tend to be faster.
(3) Considering 6D as an expensive option, what logical reason would there be to provide only one cross type focus point?
In a word: cost.
The 1D X and 5D III both have 61 point focus systems (not just 9, or 11, or 19). Some of these are single-axis points, but the vast majority are cross type (including some X type and diagonal axis points!) These cameras have highly advanced focusing systems... but of course the cameras cost more.
The 6D is a fairly advanced camera, but for as advanced as it is, it's intended to be an "entry level full-frame" camera. This is because MOST full-frame cameras cost well over $2k to start (most are nearly $3k or more). The 6D is intended to get you into the full-frame category, while keeping the price tag below $2k.
Full frame cameras tend to do MUCH better in low light (and much better at high ISO while keeping image noise very low.) This is all physics... the sensor is covered will little tiny photo-sites. These are the light-sensitive points on the surface of the sensor. When you have a full frame camera, the sensor is physically larger AND those little tiny photo sites are physically larger. When those points are larger, that means more photons on light can land on them. Think of each one as a little tiny "light bucket" and think of the photons of light like little rain-drops landing in the buckets. When the buckets are bigger... you can collect more rain-drops. Physically larger photo-sites translates into less noise.
The other nice feature of a full-frame camera is that they tend to have shallower depth of field. It turns out the true "depth of field" has nothing to do with sensor size... but since the camera has a larger sensor, YOU (as a consequence) will change which lens you use or where you decide to stand in order to get a photo of the same subject with the same framing that you could get with a crop frame camera. When you frame the subject similarly, you'll find that either you are (a) standing closer to the subject (which translates into a shallower depth of field) -or- using a longer focal length (which translates into a shallower depth of field.) Regardless... you tend to get a shallower depth of field and people tend to like that "full-frame look".
What goes against the full-frame camera is that the sensor cost is immediately higher... so to keep the costs down they tend to cheat on other components (as you've seen... the focus system on the 6D isn't quite as good as the focus system on the 70D.) Also... since the reflex mirror inside the camera is physically larger (it has more "mass" to use a physics term), that means it's harder to make it flip up and down as fast. Consequently the continuous burst sooting speed (the number of "frames per second" the camera can shoot when doing action photography) is not as fast. The 1D X has a mirror which is just as big... and it's VASTLY faster (12 frames per second when flipping up and down ... the camera can actually shot even faster still if you let it keep the mirror in the raised position). BUT... that's a $6800 camera so they don't skimp... the reflex mirror hinging mechanism and motor are heavier duty.
Consequently the 70D has a pretty fast action speed (7 frames per second) and a very impressive focus system.
There are some marketing games that companies play to "position" a product. Clearly you don't want to sell a $1k camera that has all the features of a $2k camera or consumers would have no reason to buy that $2k camera. All manufactures take some care not to create low-end cameras that eat into the sales of their high-end cameras.
Once I get your response over the above I will be able to firm up the decision. so far I am inclined towards 6D + 24-105 f4 for $ 2888 vs 70D + 18-55 kit for $1471
Thanks
Ketan
If you find yourself shooting mostly "action" photography and "video" then get the 70D.
If you find yourself shooting more still non-action photography (portraits, landscapes, or any non-moving or slowly moving subjects etc. ... only a light amount of action) -or- if you find yourself often shooting in low-light, then get the 6D.
Both are extremely nice cameras and honestly it's hard to buy a bad DSLR camera these days -- they've come a LONG way.