Both of them have pumped far too much money into branding "Playstation" and "Xbox" for them to give them up. However, I wouldn't be surprised if they don't use a number at the end. I mean would the really call the system after that PS4?
I think, much like games, the hardware manufacturers are going to move away from the "sequel" numeral, which give a sense of more of the same and go to some other addition to the brand name.
Unfortunately that probably means we'll see (<system> Next/Ultimate/XS/whatever)