How about the MSI NF980-G65? It supports the Phenom II X6 processors but you may need to flash the BIOS if the motherboard does not come with the latest one, and if you do need to flash the BIOS, then you will need another CPU in it in order to do so.
are there any disadvantages of the Nvidia chips over the AMD ones?
With an AMD CPU, if you're going to run SLI, then you need an Nvidia chipset to do it, additionally, as far as overclocking is concerned, if you plan on doing that, the Nvidia chipset does just fine, I have successfully OC'd my AMD 965 BE to a stable 4.2G on aftermarket air cooling.
As far as there being any other disadvantages, I'm not aware of any.