GTA has been a long running franchise. Crysis isn't a big franchise, so there isn't much you can compare between both.
GTA has always been an open sandbox game, from the very first game. You can go anywhere and everywhere, because it was available from the start. Crysis was big, but the thing is there's a start and finish structure for Crysis, so you can't revisit areas and such, unless you need to get something way back.
From what I can tell, GTA4 doesn't always have the best looking buildings, they do however, imitate them very well. The same applies for Crysis 2.
GTA4 has got some great physics, especially when I first experienced it for myself. Crysis 1 or 2 didn't have great physics (In terms of human bodies mainly), but there were many areas you could say the Physics exceeded.
However, both these games are my favourite (I'm a big fan of GTA, ever since the first one came out in 1998). But these two games are in different leagues to each other.