600D and 7D have the exact same CMOS sensor, so in the same optimal situation the image quality will be exactly the same.
7D offers better auto focus, less noise in higher ISO and more shoots per seconds, nothing more.
Video quality is the same unless you've to raise ISO, but 600D offers a mobile screen that 7D doesn't.
In my opinion, don't waste money on 7D and get 600D + Good lens or save for a FullFrame one like 6D/5D/1D.