I have this ridiculous theory that the quality of a smartphone is determined by the quality of the camera. However, there are other features like the microphone, display and most importantly support that should be considered as well.
Price of the monthly service!!!! I live in a good metro area, and all the major providers offer good coverage, so I dumped the majors and went with a no name company and I am saving a TON of money. I look for price, of the monthly plan, then I look for the cost of the phone. Phones are easy to come by now, you can buy good quality phones almost anywhere nowadays.
For me it will be the phone quality screen size and how it stays connected to the service provider. Not all phones connect with the same quality to the provider. Most smartphones today have the same features, some are just faster, than others.