Well i personally would go for the iPhone, because of all the apps and the looks of it (that is just my opinion)... Get the blackberry if you are more in it for texting than calling, also the apps on blackberry's aren't half bad i don't think.
iPhone all the way! It's so user friendly. I can't think of a single issue I have with mine. I'd wait for the iPhone 5 to come out though, if you currently have a phone.
The Blackberry is better for pure communication. If you're getting a phone mainly to contact other people and to stay in the know then get the Blackberry. You can type faster and it is essentially designed purely for that.
If you want a phone that does just about everything except communication better than the Blackberry, pick the iPhone. The App Store is colossal and there are so many cool apps and games to choose from. Plus it rolls your iPod into the equation too which is one less thing to carry around with you.