Corsair RMx. Any reason to not buy 850 if it's cheaper than 650?

I am wishlisting components for a build. I had kind of settled on the Corsair RM650x, but there is a sale right now, that has the RM750x ($119) and RM850x($129) as cheaper than RM650x($135)….

Corsair RMx. Any reason to not buy 850 if it’s cheaper than 650?

Source: [H]ardOCP – Corsair RMx. Any reason to not buy 850 if it’s cheaper than 650?