Currently Online 
Guess I'll be the one to buck the trend: Monster Hunter 3 wins for me.
Don't get me wrong; 4 is awesome, and in some ways the better game; there's more content, and the new mounting system is badass.
Monster wise, it's about equal; both have their share of gems, like Nerscylla, Kecha Wacha, and Seltas Queen in 4, and Lagiacrus, Barroth, and Uragaan in 3.
But 3 has better environments, and thanks to being on a console, more comfortable controls and a big screen to showcase the action.
I also prefer underwater fighting to fighting while climbing. The trick to the underwater combat in 3 is to play defensively; the lance is ideal. Wait for them to come to you, block their attacks, then punish their openings. You don't even really need to move, just stand your ground and wear them down.
