Hello, I think you should know first what you want to do with your camera.
7D is used very often to make videos and pro photography and the 600D is an entry-level dslr with good video quality too, but with certain differences in the CMOS and speed and ISO and so on.
If you are starting at photos or video, definitely buy a 600D, which is also cheaper than a 7D an it comes with a lens kit. (7D is often sold body only).