To be honest, based on Henman's pretty average display against Carlsen and Dima's thrashing of Novak...
I think Dima is going to win again
Whilst Dima is pretty hopeless on clay, his huge groundies will still really hurt Henman...and of course Henman's game is even less effective on the clay anyway. I'd imagine Tim will try to keep the ball in play as long as possible and hope for a Dima mistake...this could lead him to tire quicker.
All in all...I am struggling to see Henman win it.