OK - The "Rest of the Story"
We went with Verizon - 5gig for $50/mo. Could have gotten a similar deal from T-Mobile, but thought we would see how another carrier does, since we have voice cell phones with T-Mobile, and know that we are kinda on the fringe of good reception at the house and farm.
I used the MiFi exclusively for the 1st month, keeping tabs on data usage. We ended up using about 7 gig, so I kept the cable at the house for all the heavy bandwidth stuff. Since then, the MiFi has never busted the data cap, although we came really close this month when my wife was at the farm most of the time, while overseeing a renovation of her Mom's house. She was watching "how to" videos of tile work, flooring, etc.
I did take the magic jack along on a trip, but found I'm better off just leaving it at home. Most folks call me on the cell anyhow, and I use the cell to return messages that the magic jack emails to me (they send you an audio file). The only time I would carry it along would be if I had free wifi at my destination, so that I save cell minutes.
I also went to the cable company and asked about a lower speed plan. They gave me higher speed for less money instead. Guess it doesn't hurt to ask!
Thanks for all the good tips, advice, and helpful comments!