My firm provides me with a Blackberry, so it's all I've tried. I love it, but I'm sure I'd love the iphone too. In my opinion, there's nothing like a Blackberry for drafting business emails. So, if you're going to use the phone primarily for business, I would recommend the Blackberry, but if you're going to use the phone mostly for personal stuff, then the iphone is probably more fun and has a lot more fun applications. My only caveat is I would avoid a touch screen Blackberry, because touch screen is new to Blackberry and I hear the company hasn't mastered it yet.
I should also add that all the commercials about the Verizon network are TRUE. Verizon service is amazing.