It largely depends on the nature of the confrontation.
Wrynn is a veteran gladiator, Soloed Onyxia, Kinda taught Arthas how to fight, and has run head first into many battles and been pretty much unstoppable. I think in a gladiator style duel, Wrynn has some serious advantages.
Being an alliance player I know much less about Sylvanas, however I do see that she is a strategic mastermind, and appears to be largely unaffected by death. If it was war on the battlefield, with each leader with a number of soldiers to command, she would probably come out victorious.