I own both devices. They are both excellent!
You must ask yourself what you are buying the device for. The BB is far better for email, SMS, IM and PINning (which the iphone can't do at all.) It is also better for typing and text entry, so if you plan to enter do a lot of data and information entry on the device . . . choose the BB.
The iphone on the other hand is better for surfing and retrieving information from the web and 3rd party apps. It is better for gaming etc. If you are primarily gathering information choose the iphone.
Both devices are good phones. In a nutshell, it is easier to get info into the BB and it is easier to get info off the iphone
Do you want a phone that is communication device or a handheld computer?
In any event you will probably regret your choice
If you are on a GSM network, buy both and switch back and forth