Those processors are very close to each other in performance, almost dead even. The 950 benefits from triple channel RAM and a motherboard with more PCIe lanes, while the 870 uses less power, generally uses less expensive motherboards and has a more aggressive turbo boost meaning it will always have an edge on single or dual core apps
However there is no higher upgrade available on the 1156 Lynnfield socket (i7 870) whereas with a i7 950 you can upgrade to a hexacore later on if you want (i7 970, 980x)
Personally I'd go with the i7 870, but that's just a matter of taste. You won't notice much of a performance difference between the two