I'm no technophile, so I only know what I've read. From my understanding, you can't swap Verizon sim cards for GSM cards--again using my Lionel vs. American Flyer metaphor, they're similar but not similar enough. There's a bunch of stuff on the Internet about the differences--just Google "CDMA vs. GSM". I would prefer Verizon as well, since I've been a happy Verizon customer since they came into being--both of our phones and my iPad Mini are on Verizon.
Around here, T-mobile has pretty good coverage, and they use GSM. They have a couple of el cheapo plans, so for the limited purpose of the cellphone switch, it won't cost much to use them. ATT also uses GSM, but clients of mine who have used the ATT cellphone service around here haven't been very pleased--lots of dropped calls--so I haven't looked into using them for a cellphone switch.
Cary