so would it be better to buy the single gpu?
As a long time, soon to be former crossfire user, Imo - yes.
The 6xxx series scales better than the previous generations, so it's great when it works.
However there are still quite a few games that are glitchy or flat out don't work with 2 cards. Some of the AMD 'fixes' for this are to disable crossfire at the driver
level, or in an CAP profile, which means you'll be limited to the power of a single 6870.
In the long run it's simply less headache to run a single faster card than two slower ones.