I can't vouch for either of those two boards, but I just put together a system consisting of the Phenom II X2 555BE and an Asus M4A785-M board. As you probably know, Asus makes it simple to unlock the other 2 cores. If you want to do a quick and dirty unlock, just press "4" when the message appears on the bootup screen. In fact, I just unlocked today. Mine is now recognized as a Deneb X4 processor. I haven't tried OCing it yet because I still have the stock cooler on it. But it has been stable unlocked under Prime95's torture test for some quick runs lasting an hour or so. Paired up with my Galaxy 460GTX OC card, it provides a great gaming experience.
Looking at the two boards you mention, it would be hard to decide between them. The pro has the option of using more graphic cards in CrossfireX. But if my experience with Nvidia's SLI is any example, multiple cards seem to have problematic issues of their own. I'm a newbie at OCing, so I better refrain from giving advice in that area...