Depends on the card, if you put 2 6970 in crossfire then the loss is significant. If you put two 5770 the loss is not that big.
I also recommend the 6970. A single card always means less problems and more stability plus less heat.
+1 ; ive always liked putting one high end card instead of 2 mid ranged cards because of scaling and driver issues, etc. when they work correctly its crazy good, but i like the dependability of one card