They both perform just about the same. Stick with the MSI card because the Twin Frozr series have IMO the best cooling out there. ASUS is good as well, but if you don't have an ASUS board, the DirectCU technology won't work at its best.
Also a little fun Easter egg, you might be able to unlock the 6950. I'm currently running 2X MSI R6950 Twin Frozr III with fully unlocked shaders which perform on par with the 6970 also with less power consumption. Can't guarantee it will work for every card because some of the newer 6950's have had some of the GPU chip lasered off. Since it's a DirectCU and not a DirectCU II, i'm pretty sure it'll unlock with no problem.