That would depend on what carrier you have. At least in the US where you have Verizon and Sprint as CDMA carriers and AT&T and T-Mobile as GSM carriers. The phones are not interchangeable. If you're in the UK or some other place where there's a government imposed standard for cellphone communication technology, you still would need to figure out a way to unlock any phone not offered by your carrier.
I would look at what models your specific carrier has, and then go from there. Or if you're looking to establish cell service for the first time, you might first want to check into the different carriers in your area, figure out which one people generally consider to be the best. Personally I value the ability to actually USE my phone over almost everything else, which is why I just kind of brace myself every time Verizon thinks up a new way to shaft its customers. The charging people $30 to be able to sign a contract saying you'll pay them for 2 years was a real stroke of genius in that regard. But Verizon is the only carrier in the US that has invested the resources into building out a truly massive nationwide network. No one else even comes close to matching their coverage. So even if someone else may give better customer service, lower prices, offer better phone selection... What good does that do me if I find myself in an area with no service?
I would also give some pretty strong consideration to what Bob said. RIM is basically a dead company walking. The odds that their upcoming, and delayed, Blackberry 10 platform is going to miraculously turn their fortunes around is a very slim hope at best. I think the absolute best case scenario they can hope for, is that the BB10 platform stops the massive bleeding of users. Unless you absolutely need a Blackberry for some reason, consider an Android phone.