definitely get the 6870. performance are not that far apart and the difference in crossfire is HUGEE. 5870 scaling is terrible while with 6870 you will get almost 100% scaling. not to mention it performs better with tesselations. trust me i've owned both a 5870 CF and 6870 CF and it definitely is the better card
Second, OP is looking for a single card, not CF. So your point is irrelevant.