I have not seen a 4870 crossfire vs 4870x2 lately. I don't imaginethe difference is much, if anything.
The drivers are better for two 4870's, but run into problems in the board partner messes around with a custom bios. The two 4870's will also be cheeper.
It's really up to you. Now a days the performacne difference is nill. Get whichever you want, whichever saves money, whatever. You can't really go wrong. 2 Cards gives you spare carsd when you upgrade etc. A single card is more forgiving of your motherboard setup. Crossfire propper will have better drvier support in the coming years though, if you want to go that way. 4870's will be around as a 'budget' card long after the 5000's replace the x2 at the top of the line.