Millenicom has my vote. Outstanding customer service, too.
When we are driving down the road, I use a Wilson Cradle to keep the signal coming in strong to our MiFi. If you don't use one yet...look into it. The 3G Store will steer you to the one that works on the Sprint network. (You can use it to boost your cell phone's reception as well. It does not even have to be laid on top of it. When I put my cell phone next to the wilson cradle, I have seen the signal bars go from barely 1 to 3 bars sometimes. It really works well.)