I don't know if this is reliable enough but I always check this website http://www.videocardbenchmark.net/high_end_gpus.html
(just use ctrl+F)
The 630M has a score of 779 and the 6650M a 730.
So they pretty much equal to each other.
I don't know if the price differs, but I would go with the 630M, but just because I have a better experience with nvidia.
Also on this site, http://www.notebookcheck.net/Mobile-Graphics-Cards-Benchmark-List.844.0.html
it shows that there isn't really a difference.
So, I'd say, pick the one with which you feel the most comfortable.