The first consideration is which carrier gives you the coverage you need. The best phone in the world, if that carrier doesn't cover the spot you're in when you need a phone call, is a useless piece of electronic waste.
Check AT&T's coverage in the places you need coverage. (Don't just go to the block your office is on and see if it gets coverage. There's only one place in this entire town that has absolutely zero Sprint coverage - and that's inside my daughter's house. On the front step, on the rear deck, there's good coverage, but the only pace inside the house that has any coverage at all is one pane of one window in her bedroom. And only if the phone is held in the right position.) Radio signal propagation is an entire field, and learning the effect that causes a small space to be a hole in the coverage could take a few months of study.
See if AT&T has coverage at your desk, in the rest room you use, etc. If not, you don't want any AT&T phone. Only when you know (if it turns out that way) that AT&T gives you the coverage you need will there be any reason to have to make a choice.
Otherwise stay with Virgin and look for a better phone on their network.
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