I don't see GSP subbing Serra. He couldn't sub Mayhem. He RNC'ed Trigg out...but who hasn't?
I envision St. Pierre just over-powering The Terra on the ground, but not being able to G-n-P Serra out...because Serra is a snake on the mat.
So if I had to predict, it would be a high kick TKO...the finish will look like the Hughes fight I think.