the 6970 def beats the 5870 in benchmarks, but it also offers much better scaling performance in crossfire and imo better performance in heavy dx11 in general. Furthermore the 6970s 2gb of memory gives it better performance when higher then 1080p. But since the 5870 has dropped in price to the circa 250 range it is still a good value if you are going for a single card, but still for $300 I would much rather go with the 6950 with the good potential to unlock it to a 6970 with a bios flash all making the 6950 the best buy imo.... although at your res even a 5850/6850/6870 would be more than acceptable which are all for the most part a little bit slower (with the exception of an oc'ed 6870/5850/6850) then the 5870 so the differences are hard to quantify but I would say the six series is worth the price increase, and if you can afford it go for a 6950.. and I believe the unlocking guides are on guru3d.
also depending on what benchmarks you are referring to the information could be valid or not
what are the rest of your system specs and what games do you plan/plan to play??
these quotes overall resonate my feelings towards the 69XX series, but I feel in the end that the 2 gigs of memory will be more future proof when mainstream games ran at 1080p require more than 1 gig for high everything, or when 25X16 is basically adopted and used as commonly as 1920X1080 is today. Although I dont think that many games exist that truly test this but I feel once dx11 becomes more widely used, (not just a few renderings here and there) then the 6 series of amd will surpass the 5 series even more but for now there is not that much of a quantifiable difference.
The same argument can be made for the 6850/70 and the 5850/70 cards are all similar in performance, but the 6 series can scale much better in crossfire ans supposedly has an improved (not by much imo) tessellation system and are supposed to perform better in heavy dx11 environments being that they are amds 2nd gen series of dx11 cards. However my argument again is that I do not believe that there are many games in the dx11 arena that truly take the gpus to their limits of dx11, in fact in the dawn of the 5 series for amd it felt like they just figured out dx10 so I just think its going to be a while longer (maybe even a console upgrade) until we start seeing big margins between amds 5 and 6 series,