We looked into this 6 months ago and ended up with Verizon. I think you could get cheaper with US Cellular, but if you exceed 22GB per month, they downgrade you to 2g (in their unlimited plans). Conversely, Verizon only slows you down if there is congestion.
After a few months, my kids (teens) never go over 20 GB so that probably wasn't a real concern. My wife noticed a significant improvement in data speed (in Ames) when we switched. Also, we do tether to other devices periodically - you get 10 GB per device on Verizon.
We are currently paying for a couple phones and insurance on one. Without that, it would be a little less than $200 for 4 lines and unlimited data on all of them. I have no complaints in any of our service or performance.