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.
 

vladislav227

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Jun 14, 2011
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i want to build a new system but i want to use some parts from my old pc

i hafe a budget around 600 euro i need to buy a new gpu, cpu, motherboard, cpu cooler and power supplie

 
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?
 

badhomaks

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Mar 26, 2012
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Logically, if the 7870 had the same amount of power as a 6950 and sold for almost 40% more who would buy it? Obviously 7870 has more power assuming you can fit it in your budget.