Reason; 600T doesn't support dual fan support on the radiator, and can only be push (exhaust) or pull. Also, it has slightly more room for larger cards and is quite wide. Also has a fairly large side panel. Your choice.
I love the look of the 600t, the 650d is not a bad case, it's just to square and plain for my taste, and the 600t has sexy curves. Both cases are equal honestly though, both have almost the same amount of room (600d beats it a little) and are both nice looking. It's really up to you but I am partial to the 600t. Sorry this was so opinionated, other than personal pref. there is no better case between the two really :S
I agree with cyansnow, i own the 600t se in white, and my mates all agree best looking case they seen, has great lines to it, all room you ever need and great build quality.
650d is bit blocky for me.
If you like curves get 600t, if you like blocky case get 650d, both nice cases.