I'm currently using a Blackberry 8830 and I love it. It is MILES ahead of the Windows Mobile phone I was using up until last week, WinMo is one of the 5 worst consumer products I've ever used(I've used 3 versions of WM over ~7 years, they never change). While I think you'd be happy with both, I think each has it's strengths.
-iPhone: Given the choice right now, I would buy the iPhone. I think the software set with the iPhone is more robust than BB for my usage patterns. Personally, iPhone and Android are my two favorite platforms at the moment because of their SDKs as I would make full use of the ability to write programs for my phone (I know I could program for WM, but I'm never going there again). The browser is phenomenal for a phone, and will be even better with 3G speeds. I also like the ability to consolidate my phone and ipod into one device, less junk to carry is always good. The data plans for the iPhone are also cheaper than most smartphones, last December the iPhone ended up being cheaper than a free blackberry after 18 months due to the price difference in the plans... something to think about/ recalculate.
-Blackberry: No non-sense layout, great integration with email. One thing I like about my 8830 is that it's a world phone, not an option with the iPhone, so that might be worth considering if you travel. If you use the phone a lot while walking, or just lazy, the BB phones are very easy to use with one hand, not something that's easy on a touchscreen device. Finally, the OS has been extremely smooth and stable while I've used it- a dream compared to the reset-twice-a-day nightmares of WM.
EDIT: I apologize if I come across as a WM troll, but I'm still a bit angry and emotional from the experience I had last week. I've had many issues (mostly constant resets, losing information), but I could live with it. Last weekend was the last straw, I had to call 911 and found out that the OS crashes when it goes into emergency mode (it crashed twice in a row before they called me on a regular line). Now I've heard all the excuses about having to maintain WM unlike other phones, and I did maintain it, but I think it's absolutely inexcusable that this occurred. Would it be okay for the airbags in your car to malfunction if you didn't get your oil changed? Within 24 hours I was using a new phone.