Since you can always buy an 'unlocked' phone and choose to go month to month, some will say that you will end up paying more than if you were to go with purchasing a phone and plan together, subsidizing it over the contract plan. The biggest thing to remember if you do decide to go with a long-term contract (1, 2 or 3 years) is you are going to be paying to break it and 2 to 3 years is a long time to be with a company that you all of a sudden start to have issues with. Therefore if you do decide to go with the contract/subsidized phone plan, I would suggest nothing more than 1 year.
Got more questions? Feel free to ask me. I am always here to help!