Arlovski is faster, more powerful, better technically, and hard to take down, not that Mir's a wrestler anyway. The biggest question, as usual, is Arlovski's chin. Mir has power too and is more than capable of KOing him in the first round, or even rocking and submitting him. I still think Mir has a decent chin, especially considering the punishment he's taken in his career (I'm thinking Carwin especially), but he isn't good defensively.
Unfortunately, I think there's a good chance we see a quiet fight between tentative fighters. Arlovski has had them with Sylvia, Werdum, and Schaub. Mir had one with Cro Cop. In that scenario, Arlovski has the better odds of eking out a boring decision by being a bit more active, but Mir has a better chance of landing a KO out of nowhere as seen in the Cro Cop fight.
I think either that will happen, or we'll see a round 1 KO and it could go either way.