Bisping has a 64% career TD defense. I'm guessing that's pretty average. GSP is known for being a master of taking his opponent down and controlling from the top position. Yes Bisping will be bigger and therefore probably more difficult to take down, but it's definitely within GSP's capabilities. Plus, GSP's standup, although not as powerful as the bigger Bisping's, is more well rounded, and he can win while on the feet too, IMO, as long as he doesn't get into a slugfest w/ a guy who used to compete at LHW.
At any rate, if GSP wins, who do you think he fights next? Does he stay at MW and defend the belt? Or drop back down to WW and go for the title there? Or are we really (and not just talk) looking at a potential GSP vs. Conor fight (weight class TBD)?
He will probably fight Brock Lesnar or Matt Hughes.