The Boost is a faster card and 2GB of memory is almost mandatory these days. Plus with the Boost, you can enable PhysX in some of the upcoming games like Call of Duty Ghosts, the Witcher 3, and Batman Arkham City.
Definitely go with the 650ti. It has more GDDR5 memory and on benchmarks it looks better than the 7850, plus it's cheaper
Its quite clear that the 650ti is having 2gb ddr5 while the radeon one is just havin 1gb ddr5. Hence the 650ti has more Vram enabling u to play games on a bigger resolution.