If you're choosing between a Nikon and Canon and they offer the same features then the only way to know which is best for you is to ask to try them out (if you're viewing them in a shop), see how it feels in your hands and how you like the controls. It's exactly what Voyager said, it's all about the feel.
Differences in image quality between different brands of DSLR no longer exist, so ignore anyone who tells you to pick camera X because the image quality in camera Y is poor... unless camera Y is some shoddily made off-brand or is actually much older then they're just fanboys.
I should add that I'm not a gear-head, I know literally nothing about the 600D or D5100 and wouldn't even be able to tell them apart if you took the branding off, but this is the general advice I give everyone because there are a lot of gear-heads out there who will insist that Canons produce better images than Nikons or vice-versa