In battle Spaz is deffinetly better, it is muc easier to move with him. (he can just jump away from a firefight, meanwhile Jazz takes forever to first get in position, then crouch, then jump, and if you miss the ledge, frankly, it can just get annoying)
But, in different modes, such as treasure hunt, or assasination or tests(bleh! ) Jazz has a definite advantage. In race they are about equal with different situations being used. Now Lori, to quote everyone else: just sux! Capture the flag varies by lvl. Battle is most played, especially if you join a N00b's server (which are everywhere) so it is usually easier to play as spaz. Spaz is also better at dueling, I think. (now all u duelers who play as jazz *looks at himself* don't get mad! *commits suicide*)
Anywayz normally Spaz has the advantage but I like jazz better anyways, but sometimes I switch to spaz for battle or something. For single player I also always play as Jazz, 'cause even though Spaz could come in handy, it is not fast paced, so Jazzes moves, which slow you down, are much better.