You should always pick the carrier first. Look at where you will use the device - where you live, work, travel, play etc. Check the coverage maps, but don't believe them. Talk to friends, family, co-workers, etc,. and see what they use and how they like it.
Since you didn't say where you lived, it is tough for anyone to make a suggestion.
Once you pick the best carrier, then pick the best phone for your situation.