The 650 gives you better support for DX11. I would also suggest the 650Ti the 650 is not a great value in terms of a graphic card. You will get quite a bit more out of the Ti the 650 on its own would be good for a home theater build with very okay settings within games you can turn settings up a little more with the Ti. The main con I would say of both cards is that they are poor gaming cards.
do you have any links to those? the GTS450 is quite an old card while the GTX 650 is much newer. without really chasing it up, I'd say the GTX 650 is better, by a noticable margin. but like bigshootr8 said, they're not really gaming cards.