...
Space being a concern for you, however, means 5970's are a better buy. They're also more widely available, 5870's are a bit hard to come by these days.
5970 being easier to find than 5870 ?
Dont' think so.
If you are in the US and willing to buy used (ebay has a 30 day guarantee) there is essentially an unlimited low cost supply of 5970s. Yeah I know some people are afraid of buying used GPU but 30 day return policy cuts the risk. If some fail you have to factor that in compared to the higher price of new units.