I own both devices (Bold and 3GS), and periodically I switch devices when I'm in a business mode or leisure mode. As we know, the iPhone kicks ass with its Safari browser, but nothing beats a physical keyboard along with a replaceable battery and MicroSD memory card.
Why I like the iPhone: Safari browser, and the fact that chicks are more impressed with the iPhone when I go out clubbing with it. Chance of getting laid is easier when you sport the iPhone because chicks dig it more.
Why I like the Blackberry: easy to switch the MicroSD memory card. You can put in all the nature pictures, romance flicks and songs in the device when you are with your girlfriend, and while she's away you can put in the MicroSD memory card loaded with nasty MP4 pornos for fun. Try doing that in your iPhone, buddy!