Well, the huge amount of vram on the GTX 960 is sort of dumb since it's pretty useless because the GPU isn't powerful enough to use all 4GB of it.
However, the GPUs themselves are definanlty different. the GTX 960M is noticeably faster than the 950m. This is especially true if you get the gtx 950m with DDR3 vram. The GTX 960M comes standard with GDDR5 which is WAY faster than DDR3.