I've been shooting long enough that I don't get wrapped up in the Canon vs. Nikon debate (or frankly any debate). I shoot with a Canon, but I think any "capable" camera is a good investment and there are lots of them on the market. Your own skill, followed by lighting, followed by lens choice (& qualities of the optics) all have a much larger role to play in the quality of the image you get out of the camera. In my own opinion... the camera actually finishes in last place (well... this assumes the camera meets some minimum capabilities. I can imagine having a camera which is inadequate for a task. I wouldn't want to shoot action photos & sports games with a point & shoot that had a serious case of shutter lag.)
If you've got a healthy investment in Nikon glass and other brand-specific accessories then that might be a good reason to stick with Nikon. They DO make great products after all (and yes... so does Canon or I wouldn't be using them.)
You mentioned that if "Nikon would offer everything in one package like Canon" that you'd go with them. Can you give us an idea of what types of features you are looking for and what type of shooting you like to do (what are your typical subjects? Portraits? Landscapes? Sports games? Architecture? Macro? etc.)
As for the Canon lineup...
What you need to know is how they position products in their lineup. In the US the bodes are sold as "Rebel T2i", "Rebel T3i", and "Rebel T4i" (which corresponds to the European model numbers 550D, 600D, & 650D). They also have the Rebel T3 (without the "i" suffix) which is the 1100D in Europe. The 1100D is considered the low end of the entry-level category for Canon DSLRs. It's the most affordable but also the most basic. The "Rebel" bodies get a rev just about every year (pro bodies get updates less frequently... often 2, 3+ years between updates). The 650D is currently the high-end of the consumer/entry-level bodies. The 600D was the top of the line... "last year". The 600D had that spot the year before.
Canon has not increased the resolution of their APS-C sensor in a while. The 550D (which was a jump over the 500D), 600D, and 650D all use the same sensor. Also, the 60D (a pro-sumer body) and the 7D (arguably either a high-end pro-sumer body or some might say a low-end pro body) ALSO use the same size and resolution sensor. While the firmware changes and sensor tests can show very slight differences, better noise handling & ISO, but it's not really much. MOST of the differences are in all the "other" features of the body besides just the sensor.
The 650D has a flip-out screen which is ALSO a touch-screen. (the T3i has a flip-out screen but it's not a touch-screen). The T4i has auto-focus during video. It has a Digic V processor (much faster than Digic IV -- the T2i and T3i have Digic IV). One of the biggest features of the T4i is the focus system improvement... previous "Rebel" bodies had a 9 point AF focus system where only the center point was the fast/accurate "cross-type" focus point. On the 650D ALL 9 points are now "cross-type".
The T4i/650D is now so good that it competes with the Canon 60D (so we all wonder if they'll have a 70D on the horizon). The 60D still uses a Digic IV processor (which was the most advanced when it was introduced a few years ago) but the body is weather-sealed (that's not "water proof" ... it really just means they put seals on all body seams, buttons, and dials have o-rings to keep out water that might spray, rain, splash, etc. on the camera ... as long as you don't submerge the camera. Of course you'd also need a weather-sealed lens to complete the seal.) All 9 AF points on the 60D are also "cross type". The 60D has a control layout more similar to Canon's pro bodies. That means you have a top LCD screen (in addition to the back screen) showing you most of the exposure and drive settings.) In addition to the front selection wheel, it also has a rear-selection dial. In "manual" mode you can use the front wheel for shutter speed and the rear dial for aperture. This allows you to use your index finger and thumb simultaneously to quickly dial in exposures settings on the pro bodies (that would have required pressing combinations of buttons on a "Rebel" body.)
The 7D is optimized for sports. It has DUAL Digic IV processors (all lower bodies only have 1 processor). It can burst at 8 frames per second. It has a 19 point AF array and ALL 19 points are "cross type" (very fast... very accurate.) It uses CF cards rather than SD cards because CF cards traditionally have MUCH faster I/O speeds than SD cards (although the latest SD cards are catching up.) Like the 60D it is a weather-sealed body, but the 7D also has a magnesium alloy body (metal) and is much more durable (and physically heavier) than the 60D and lower bodies (which use polycarbonate (plastic -- although it is pretty tough.))
There are strong rumors that a 7D II may be on the horizon ... possibly in January (don't take that to the bank.) Some fairly attractive deals for the 7D have been showing up. These may get even more attractive if a 7D Mark II is announced.
The 7D is basically the top of end of Canon's APS-C crop-sensor size bodies. Above that you have to go to full-frame (6D, 5D II, 5D III, or 1D X -- they no longer actively market the 1D IV or 1Ds III)