Sales of the RT have been relatively sluggish against other tablets. The Pro will be competing with a wide range of Win8 capable notebooks and ultrabooks and its specs don't really offer much of an advantage over, say, Dell's or Lenovo's offerings in that range.
For those reasons, I doubt the Pro will be a big seller. But they will sell some -- Microsoft has it's share of "fanboys" too, who will jump at it. It seems a capable enough machine (battery life questions aside) so some people will choose it over other offerings.
Just a prediction, and as I noted upthread, predictions about the tech industry aren't necessarily reliable, historically speaking.
All the 32GB Surface tablets sold out on the first day they were available for pre-order. Given that the platform is relatively new in the market and most of the hardware offerings are still in development I think it might be a little early to be comparing sales numbers based on the OS platform.