Since performance is practically identical I would personally go for the 5970, simply because it takes up less space. There are probably other considerations I'm unaware of though.
also the price for a single 5870 is around 400 bucks, while the price for a 5970 is 600..so it's better to buy the 5970 if you plan on running two 5870s anyway, it'll be 200 bucks cheaper and you dont have to hassle with linking them