I have a 5D II and now a 5D III. A 7D is known for it's snappy performance in action photography. A 5D II would be completely different (as would a 6D). These offer full frame -- so you'll get the new angle of view, a larger brighter viewfinder, you'll find that when capturing the same subject with the same lens you'd now stand closer to get the same framing and thus reduce the depth of field and end up with stronger background blur (all things people tend to like). Another side-effect of full frame is the naturally larger photo sites on the surface of the sensor. Physically larger photo-site tend to result in lower "noise" when shooting at high ISO. Consequently you'll be a bit bolder about shooting at higher ISO than you would have done with a 7D.
But the focus system on a 7D is is better than the focus system on both the 5D II and 6D (but not better than the 5D III -- which is among the most impressive focus systems currently on the market). The 5D II has a 9 point AF system in which the center point is the only cross-type point (there are some focus-assist points you cannot control). The 6D has an 11 point system. It looks nearly identical to the system on the Rebels and the 5D II except they squeeze in two extra focus points ... one left and one right of the center point (both inside the 8 AF points in arranged more or less in a "diamond" pattern.) Again, only the center point is "cross type" even on a 6D. The 5D III has a 61 point AF system and 41 of those points are "cross type" and it's highly configurable and tunable to the shooting needs. It's sophisticated enough that anyone buying the camera really should take the time to sit down, read about the system, and learn to use it.
The 7D will also have a faster frame rate than all of them (the 5D III is fairly quick... just not as quick). It's difficult to have a very fast shutter on a full frame camera because everything inside is physically larger so you're throwing more mass around -- although the 1D X certainly does it. The 5D III's focus system and fairly fast shooting speed are fast enough that people will use it as a sports camera. You would probably not want to use a 5D II or 6D as a sports camera (though technically a person could probably rationalize using any camera as a sports camera -- so let's just say there's nothing about those cameras that would suggest they've optimized them for action photography or sports.)
Of the three (5D II, 6D, and 5D III) the 5D III is certainly the best of them (and really there's not much of a comparison... the 5D III is built better and outperforms in nearly every way.) The 6D adds the GPS and WiFi (although the 6D GPS isn't as good as the hot-shoe attached GPS module that Canon sells.)
As for lenses...
The EF 24-105mm f/4L IS USM is the lens often referred to as the "kit" lens... but this is an "L" series lens (don't think "kit" lens as in what would be included with an entry-level camera.. the 24-104 is a very nice lens.) It can provide f/4 throughout the focal length range and when you shoot full frame you sort of get a narrower depth of field (in reality you don't... you get a wider angle of view which means you'll stand closer to get the same framing and that "closer distance" is what causes the narrower depth of field). As such... I tend to not dip below f/4 very often anyway. I think an f/4 lens with a 24-105 focal length range is actually an excellent choice. If you could only have "one" lens... that'd be the one.
With that said... I don't own that lens. I knew I was going to buy the 70-200 f/2.8 in fairly short order... so I went with the 24-70 f/2.8 knowing that my focal lengths were covered, I wanted the lower focal ratio, and I didn't care about that the image stabilization (the 24-70 is not image stabilized but the 24-105 is).
I bought my 24-70 and 70-200 for my 5D II and that means I have the first generation 24-70 f/2.8L and the 70-200 f/2.8L IS lenses -- both of which have been replaced by 2nd generation versions that are even better (not the first generation were slouches ... they've very nice... but the v II lenses are even better.)
That is to say... IF you felt that you were going to get a 70-200 fairly soon... then perhaps you might consider the 24-70. But if not... go with the 24-105 as it's more of an all-around versatile lens.
The prices are not even remotely close to each other. Canon sticks an $1150 suggested retail price on the 24-105... except that same lens is the "kit" lens when you buy a full-frame body+lens kit combination. The lens tends to add about $550 to the price tag of the body-only. Consequently, it's very easy to find the 24-105mm for about $600 or perhaps even just a tiny bit less if you shop around (it might be a "white box" lens meaning it was the kit lens that some buyer didn't want so they unbundled it.) This assumes you buy a "used" camera that has no lens. If you're buying "new" then you could just go with the "kit" and get the lens for about $550.
If, however, you want the 24-70 f/2.8L II... that's not cheap. That's $2300... and while there's currently a $200 mail-in rebate... it's still over $2k after the rebate.
So when faced with the 24-70 f/2.8L II for about $2100 vs. the 24-105mm f/4L IS for about $600... you've got to REALLY want that f/2.8 aperture to fork over an extra $1500.
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