From both video cards, the GTX 570 is better.
But a 570 for just Sjyrim seems like an overkill, unless you plan on playing other demanding titles.
Also, at what's your monitor's native resolution?
They are surprisingly easy to achieve. You can also go to http://www.systemrequirementslab.com/cyri/intro.aspx
Which is a great place it will automatically scan your hardware (With perission of course) and it will tell you how your system will hold up to Skyrim!
Go to THE Elder Scrolls Skyrim, I messed up and went to just Skyrim and couldn't find it.
A 6950 or a gtx 570 Are perfectly fine (from what I'm guessing) for Skyrim at 1920x1080
-Mithness
I am not sure it needs that much of a powerful GPU, not Skyrim. I could run much more demanding games on a 5770 at 1080p, with most of the details set to max.
And still, I will get me one of those 6950's soon.