I have owned both a blackberry and an iphone. I currently own a Nexus S. You can see my review here http://hubpages.com/hub/Moving-to-Android-Nexus-S-...
The answer depends on what you want to do with your phone. If you are really only interested in email and a phone, I think the blackberry is hard to beat. The keyboard is well laid out (and the expense of the screen) and the device is well designed and fast for those applications.
However, if you are interested in applications like camera, twitter, facebook, browser, angry birds - whatever - there is no choice between the two and its a very easy decision to go with an iphone (or android device). The blackberry browser is terrible (still) and the application ecosystem is really lacking since RIM for whatever reason has done a poor job of making developing on their platform easy for a variety of reasons that are beyond the scope of the question.
Hope that helps!