I'm under the impression newer cards are voltage locked, though I haven't really read into it beyond the odd notice in threads here as I got one ages ago. For new voltage locked models, I assume the potential overclocks are seriously limited and a card with a less good cooler but voltage increasing ability would be a card to go for over one with a great cooler but limited overclocking potential.
You can always put water/3rd party air cooler on a crap card with overvolting ability, while the other card will always be limited.
Depends what you want to overclock to, what you want to spend, how you might want to cool, what noise levels you want to put up with.
I'd have gotten an old windforce with voltage control, if they don't have voltage control now, I wouldn't.
A 6950 to a 7950 is a fairly large jump in performance, but my general advice is, in any given generation buy within the first few months to get the most "value" from it, the last few is the worst time to buy. With probably a 8970 due in 3-4 months(maybe sooner), I'd hold on, and get a price dropped 7970/50 when the new cards launch of see if the new cards offer better value and the better option. I'm not expecting anything magic from the 8xxx series cards at all performance wise , more that if you can get 15% more performance at the same price, or if the 7970/50 drop by 30% in price to clear, then good options all around.