*I s'pose there's Dark Brotherhood spoilers ahead. You've been warned.*
This one's definitely much closer than the first. I'm gonna throw a vote behind Cicero, because he's already taken down a werewolf, and sure, he was injured severely, but as far as we know (or as far as I remember, if he mentions something to the contrary) that's the first werewolf he's ever even fought. If he can take on a werewolf without even preparing for it, imagine what he can do now that he knows a little more about their strengths/weaknesses. It also seems likely to me that Arnbjorn has been a werewolf much longer than Aela, and was probably far more experienced in using that form.
But still, Aela's got range with her bow, and a werewolf's still a werewolf, no matter how high pitched your laugh is. So perhaps it depends on what stage in the game the fight takes place - before/after Arnbjorn and Cicero's fight.