I'm a Go surfer but i agree with Hugh, u soon wants something with just that exwtra edge. The go, on flatwater is more like a racing board than a real freerider. Carves offer u that free feel you're looking for and the 133 gives you the possebilety to put your straps inward aswell as outward so for any conditions ur sailable (the Go doesn't have this, a shame though, it only gives u beginner, medial and external setting). The 133 would be the better option if i were you. I always say, take the board with the least amount of liters possible for your level so u can experiment on rougher conditions with it aswell. The Go 139 and Carve 133 resemble eachother in most aspects (go to the 2006 starboard website, they make a professional comparison on it between the carve and the Go).
Have fun and good luck!!
(oh, on my avatar i'm surfin the Go 139
Haiko aka Crazychemical