As others have said, try on as many as you can before making a decision.
I will add: Find a large tack store that carries as many different brands as possible. A Dover Saddlery or the like; most farm stores and smaller tack stores only carry a limited selection. Also, _don't_ shop by price tag; don't even look at it until after you've tried the helmet on. Your cabeza is way more valuable than any helmet on the market, and a comfortable well-fitting hat is far more likely to be worn consistently than a funky "down on your nose" one. And a helmet just doesn't do any good at all hanging on a peg in the tackroom.
Also (and I preach this line whenever I get the opportunity), look into a riding vest. I really _really_ like my inflatable vest. Expensive, yes, but if it saves just one trip to the ER, it has paid for it's self (actually, the same goes for the helmet). I'm OLD, and I figure if the vest/helmet save me the pain associated with just one landing in The Dirt, that's enough in it's self :-)
And I can personally testify that the inflatable vest works, and works well. Ask, and I'll tell you all about it.
Steve Jernigan KG0MB
University of Colorado