That depends on your needs, location and what you consider a a good deal. I have very small amounts of cell and data for nothing a month to full blown unlimited data hotspots with roaming for $20 a month.
Location
I can tell you that from Denver to the South west that AT&T and Verizon are very close in coverage unless your plan does not have roaming, then get Verizon.
Unlimited talk and text isn't a big deal anymore and a number of plans offer unlimited data, On the cheap end of the unlimited data is the Verizon 3G hotspot for $5 a month. Above that I'd have to look at the deals out there now as they change constantly.
So you have taken the first steps, lets narrow down your needs out of the many different plans that might fit them. It is not just the main carriers to consider but a slew of MVNO's that resell the services cheaper. Things to look for is if the unlimited data is really unlimited, is there a hotspot option, does it cost more, is it unlimited in data and speed. Are taxes and fees included? Does it roam?
In the communication section you will find a bunch of post dealing with cell service, not all of the deals are active but new ones are listed all the time.