In terms of surfing websites, the iPhone is remarkably better than any Blackberry yet (dunno how much better the Storm will be).
In terms of texting, I'd have to say it's at best a wash... I actually liked this slightly better on the BB.
In terms of e-mail, I don't see any reason you shouldn't be able to receive both .Mac / MobileMe mail and GMail on your Blackberry. But you do definitely get rich text and HTML mail on the iPhone, which sounds like it would be a major plus.
Changing networks? People vary on this, but from what I've seen, a comparable AT&T smartphone plan is about $30-40/mo more than it is for T-Mobile... meaning, inclusive of taxes, I pay $70 now and as closely as I can estimate, I would be paying about $100-110 for comparable services on AT&T. And, of course, sign on for a contract again.
My choice was to stick with T-Mobile and use an unlocked iPhone with EDGE. That might be an option for you... For me, the money was a significant component -- I'd get 3G if I went with AT&T, but ... here I spent $250 for my phone (refurb) and $70/month; with AT&T the differential costs (+$50 for a new 3G iPhone, say +$30/mo for the contract) would be well over $700 in two years.
If the monthly charges aren't an issue, though, I'd probably get the iPhone 3G if I were you....