Here is an in-depth explanation of my experience with a Hockey Style Helmet.
http://www.reegind.com/helmet.htm
Mask vs. Helmet, gets about as heated as MAC vs. Windows.
Best thing is to try both.
When I say try both, I mean do an entire game with each. And, if it doesn't happen during the games, get someone to hit you square on the nose with a fastball wearing each. Mask: Ouch, what's that ringing in my ears ? What are those stars doing out in the middle of the day ? Helmet: Did a mosquito just land on me ?
BE YOUR OWN JUDGE !