The cool thing about the Ting is that I'm not screwed either way if I want to use more. And this hasn't happened in a few months, but if I needed to, say, use more data or minutes in any given month, that's fine. I may then pay $26-29 or whatever, and I'm good with that. But again, that hasn't happened in awhile.
Also, even when I leave data on, I can still stay below that 100MB, but it may get harder in time with the way data is going on smartphones. For you with the data needing to always be on, then T-Mobile may be the way to go. That was may 2nd choice after Ting honestly. The other thing I really like about Ting is that they have the absolute best customer service I've ever encountered with any phone related company. They just seem like genuinely nice people that are looking out for your best interests.