How much data do you feel comfortable having? If you could be on wi-fi all the time, then 200MB on AT&T(or 300MB on Verizon) would be fine. Otherwise, you would need 2GB of data.
Sprint is typically the cheapest for the money unless you're getting the smaller data plans on the other carriers. Really up to you at that point though. I'm on Sprint personally and don't have any problems with data speeds/service. I'm in California, but travel frequently to Seattle as well if that helps you any.