There was a comprehensive thread on this a while back:
The supported frequency bands need to match what’s available in your area. Take a look at the first post on that thread to see which ones Verizon and T-Mobile support. You can also look at cellmapper.net to get a map of where certain frequency bands are used.
In general the Broadmobi 818-A1 seems to have the best coverage, but the Gemalto PLS8-US should still work. I’ll be going with the Gemalto modem as it’s made in Germany rather than China, but if country of origin is less important to you then you may find the Broadmobi modem more appealing.