You can probably tell that I love my iPhone, and if you find it easy to use, it sounds like a good idea to me. However, I don't want to bias you unfairly, since there certainly are some people who like other types of phones better. I actually don't have much personal experience with Blackberry phones myself.
The $15/month plan is a data plan that is only available for some specific phone models (feature phones, which are generally less sophisticated than an iPhone or Blackberry).
With the way phone subsidies work, unfortunately being an existing customer can sometimes make the price go UP (at least for the phone itself), but I'm not aware of any discounts offered to existing users. Usually, once a cell phone company has you locked in with a contract, they may stop caring about impressing or satisfying you.