sorry i hit enter before i was done. Strictly from a supply perspective the 5870 will be much harder to find if you can at all in 6 months. performance is very similar with the 5870 being a little faster all around as a single card. CF is a whole different story. 6870s scale very well in CF so performance should catch up if not exceed that of 5870. 6870 is also a physically smaller card and consumes significantly less power.
and you will be able to find the 6870 for some time still, 5870 not so much