That's a tough one. Like my bo Freak said, Monson is a beast, but at 6 feet, he's considerably smaller than Tim.
I think Monson's got Tim beat in the overall skills department, but I also feel that Tim's reach advantage would be too much for him.
My pick: Tim. Having said that though, I don't know what implications the fight will have because I think that Arlovsky will beat Tim at UFC 61.