In term of mobile graphics, I think it goes something like GTX 560M > Radeon HD 5870 > GTX 460M. The GTX 560M has 1920 x 1080 resolution while the Radeon HD 5870 has 1600x900 which makes everything small..
nVIDIA has slightly better driver support, CUDA, and PhysX. In my opinion, PhysX is not that important but it's up to you. Just decide if a slightly better graphics, 1920x1080p, 2gb ram, and usb 3.0 is worth the $150 increase