Generally speaking 5-10% performance difference is not noticeable in the real world. Therefore, you really will not be able to tell the difference between these 2 cards on average.
Where you will be able to tell the difference is in the games where 1 camp is faster than the other (and when NV is faster, it's really much much faster - CIV5, Mafia 2 PhysX, Hawx2, LP2). I mean really in the other demanding games you will be running 42 vs. 46 fps (i.e., both well below 60) or in less demanding games, way above 60. That's my point. Neither of these cards is going to produce a better gaming experience today on average.
Personally, it sounds like you don't want the hassle of unlocking/overclocking the 6950. Even if you unlock, that only nets you a ~5% performance gain. So taking unlocking out of the equation, it sounds like you are more comfortable going with a faster card out of the box or a card that will get the fastest performance once overclocked to the max at 1080P - that card is the 570 if you don't care for the $60 to get there.
No one knows what will happen in 1.5-2 years, but it's almost guaranteed that no one will care if the HD6970 or the GTX570 is faster. By that time 28nm refreshes will be in full swing. Just to put 2 years in perspective for you:
1) August 15, 2009 I bought an HD4890
2) June 15, 2010 I bought a GTX470
3) February 21, 2011 I bought an HD6950
This means in only 1.5 years the market went from an HD4890 ~ $175 or so to an HD6950-->HD6970 ~ $230-250. That's about a 50-60%+ performance increase (and way more in the latest games, use an HD5870 for reference). So in another words in 1.5-2 years the difference between a GTX570 and an HD6970 will be more or less nil (i.e., too slow in new modern games by then to max them out). This is why I am advocating just putting the $60 into an SSD or put it aside for the next videocard upgrade.
Where you will be able to tell the difference is in the games where 1 camp is faster than the other (and when NV is faster, it's really much much faster - CIV5, Mafia 2 PhysX, Hawx2, LP2). I mean really in the other demanding games you will be running 42 vs. 46 fps (i.e., both well below 60) or in less demanding games, way above 60. That's my point. Neither of these cards is going to produce a better gaming experience today on average.
Personally, it sounds like you don't want the hassle of unlocking/overclocking the 6950. Even if you unlock, that only nets you a ~5% performance gain. So taking unlocking out of the equation, it sounds like you are more comfortable going with a faster card out of the box or a card that will get the fastest performance once overclocked to the max at 1080P - that card is the 570 if you don't care for the $60 to get there.
No one knows what will happen in 1.5-2 years, but it's almost guaranteed that no one will care if the HD6970 or the GTX570 is faster. By that time 28nm refreshes will be in full swing. Just to put 2 years in perspective for you:
1) August 15, 2009 I bought an HD4890
2) June 15, 2010 I bought a GTX470
3) February 21, 2011 I bought an HD6950
This means in only 1.5 years the market went from an HD4890 ~ $175 or so to an HD6950-->HD6970 ~ $230-250. That's about a 50-60%+ performance increase (and way more in the latest games, use an HD5870 for reference). So in another words in 1.5-2 years the difference between a GTX570 and an HD6970 will be more or less nil (i.e., too slow in new modern games by then to max them out). This is why I am advocating just putting the $60 into an SSD or put it aside for the next videocard upgrade.
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