John,
I have both a Verizon MiFi (3G version) and a AT&T 4G USB (which I connect to a CradlePoint device). I have two different devices because when I travel for work, I have to have Internet connectivity in the field. The Verizon MiFi works 99+% of the time, but sometimes I will not have a strong signal. So I will turn to the AT&T device to see if I can get a better signal.
Up until a couple of months ago I had a Sprint MiFi, but I finally abandoned the device (along with my Sprint service) for poor performance overall during the past 2+ years.
I will typically use speedtest.net to get an idea of what my bandwidth looks like with each device.
At the CG, I will see what their connectivity looks like as well. I bought the wireless repeater from Jefatech for my rig, which I found to be a nice addition for boosting the CG WiFi signal.
I have found the Verizon MiFi to be most reliable out of all the service providers (BTW, I do not work for Verizon).
I have had to turn it off and back on every couple of days to reset it. The device would have a green light, but no real connectivity.
-Bruce