Power supply's that come with cases are usually low-quality and cheap.
That Antec is OK, but it is a captive PSU so all the cables are hardwired to it, you'll need to do more cable management. I'd stick with the XFX, though if you really want to save a bit of cash and forfeit the cool green fan then the
Antec TruePower is also an excellent PSU. However, the saving this gives is fairly small, and if you want to really cut down the price on the PSU, you should go for the 650W version of the TruePower. This will power your system fine, but Crossfiring two 6870s would push it near it's limits. Remember, just because a PSU says 650W doesn't mean it can give 650W.