This is a pretty good fight; both of them are pretty powerful, and I think I agree with Leonidas. So, basically, I guess this match is a tie, sort of - but if you have to go with one winner, I guess in a sort of tie breaker (since Thor wins on land and Namor wins in the water) I'll go with Namor, if only because, as battle-weary as Thor is, Namor just seems craftier.
Of course, Namor is such a situational character, it's hard to call any fight with him in it down the middle - which is a big reason he's so interesting, I suppose.