Depends on what you are looking for! I have a blackberry and I'm very happy with it. My friends who have iPhones are also very happy with their phones! I'd like an iPhone, because it takes more apps, but blackberry messenger is really great*
Unless you find the physical keyboard important, go for the iPhone. If a higher number of your friends have one of them, go with that when since they each have their own free instant messaging service. In overall quality, the iPhone is better. There is also an app called kik which lets you IM your friends that have iPhones, Androids, Windows Phones, or Blackberries. I would also recommend looking into the Nokia Lumia 920, a windows phone coming out soon.