950 is better, it mantains almost the same fps in Metro LL at very high settings than the 670 in Metro 2033 at high settings with 4x MSAA (probably you can quit the MSAA and set to very high textures and have the same fps), both @ 1080p. Also it has better memory compression, so it would be valid to say it has more GDRAM.
However, overall gains will be 10% at most, so it won't be worth buying a 950 if you already have a 670. If you can buy both of them at the same price, of course the 950 is a little bir better and uses less power and newer coolers, so it will be much more silent.