Just wanted to add my 10 cents. I was on a very similar plan (maybe the same) as the OP until last Christmas. I also live in the Greater Boston Area, and figured that T-Mobile's service would be perfectly fine for me 95% of the time. Unfortunately, I can't use wifi at work, so I did want more then 500MB of data. I got on Solavei, which resells T-Mobile service. $45 a month for unlimited talk, unlimited text, and 4 GB of 4G data, with unlimited 2G afterwards. I have been very pleased with it so far, and when I was visiting my parents in Jacksonville, AL over the holidays, I actually found that I had better reception with Solavei than I had with my old Galaxy Nexus on Verizon.
I brought my own Nexus 4 to Solavei, but at 300, that phone was the same off contract as some other phones are priced when on contract. I know you can purchase phones through Solavei, but I don't believe they have any sort of monthly payment plan for them. For those curious about T-Mobile's network but not wanting to have only 500MB of data, I highly recommend you at least give Solavei a look.