Well , simply put , the actual compute power of the GPU in this scenario is what will limit performance and the game will often hit these restraints before tapping into the entire memory buffer on either gpu. Not to say that Memory speed and bandwidth have no effect becuase they certainly do. The caveat with that is memory performance will stretch its legs on higher resolutions , the 850M while only 32GB/s memry bandwidth , to the 80GB/s (or so) will likey not be the hardware limitation with many , many titles. I would think the higher shader(ALU,CUDA Cores,SP..) count and texture unit count will get those pixels moving just as well or better than the desktop GTX 650
I would go for desktop definitely, because you can do an upgrade any time you want. Also it depends on ram and cpu that you are going to have. As for laptop gaming I would not recommend because all the components that manufacturers use there are mobile and weaker than in desktop solutions. So choosing desktop you will get an upgradable piece of hardware, also you still can play your games in full HD unless your laptop have a 1080p monitor which I doubt. So go for desktop and you will be happy. My close friend is really unhappy with his top notch laptop (4 years old) with mobile graphics card and core i7 with 8 gb of ram. So we are going to buy a budget desktop for him in few months. Only advantage of buying a laptop here is a gaming mobility.