The 7870 is the faster card.
BUT, are you thinking of purchasing one of them for a new build, an upgrade for an existing system, or just curiosity?
For a new build, the 7870 is probably the best choice.
for an upgrade the 6950 is likely to be the better choice but we'll need to know more details of the system you're upgrading before we can make a solid recommendation.
That's a little harder.
Frankly I'd o for the 6950: It's a whole hundred Euro cheaper and that money might be better spent on a faster CPU and quality powersupply rather than gaining a fairly small performance advantage over the older card.
Off topic: Have you considered the 7850?