I have to go with BI. While I would say the MTW battle AI was better overall, there are SO many features in RTW and BI that enhance the gameplay. The MTW strat map existed merely as a method for arranging the battles. Just the extra dimension of gameplay that results from the beautiful RTW strat map is more than enough to put it over the top of MTW.

I'm now very eager to see what the next TW installment brings. Hopefully the best of both.