Welcome to the forum gbak.
First off, if I can play farcry4 etc quite happily on a 4gb system, I do question whether it is really the ram which is the issue given the types of games the surface lines are capable of playing (i.e nothing high end). What is it exactly that you're trying to play.
Secondly, I have to admit that it seems an odd question. The surface book and surface pro are very different machines, so seems odd to be deciding between the two purely for gaming purposes. What is the main function of the machine for you? That should be the deciding factor, with gaming as a secondary consideration.