I would say Go for Blackberry. Bold is an excellent phone.
You will love its keyboard and the blackberry's known for its e-mail handling.
The iPhone does the job well enough, but I think having used a BB before, you will miss the push-mail and the BB-Pin messaging.
I'm not 100% sure, but i think iPhone does not handle e-mails the way Blackberry does. In a BB, emails are received the way we receive SMS. The moment an e-mail comes into the inbox, the phone downloads it instantly and we get a notification.
In the iPhone, the e-mail settings are stored on the device, and it regularly syncs it with the mail server. If a mail comes, it would be downloaded on the device on the next sync cycle.
That is the way e-mails on my HTC work. It's not really bad, but if I set the sync interval too soon, it drains the battery. Blackberrys are created keeping this in mind, and email connectivity certainly won't be a cause of excessive battery usage.
Another con for the iPhone - Typing would be a pain. Especially after you've used a Qwerty keyboard on a BB.
I'm not highlighting the Pro's, because they have already been mentioned in this thread.
Good luck with your new phone