Choosing can be a hassle can't it.
Sprint, Verizon and U.S. Cellular use CDMA. AT&T and T-Mobile use GSM.
Well straight talk actually has something to help with choices. You can check comparability and location by going here...
https
/www.straighttalk.com/wps/portal/home/h/BringYourOwnPhone/carrierselection
For call quality, the technology you use is much less important than your carriers network. There are good and bad CDMA and GSM networks, but there are key differences between the technologies.
What most people don't realize is it is way easier to swap phones on GSM networks. Why? Because GSM carriers put your information on the removable SIM card. So all you need to do is take the card out, put it in a different phone, and the new phone now has your number.
I actually did that myself, when I was still using AT&T.
Plus, to be considered GSM, a carrier must accept "any" GSM-compliant phone. So the GSM carriers don't have total control of the phone you're using, leaving you more options.
With CDMA, however, that's not the case. In the U.S., CDMA carriers use network-based white lists, rather than Sim cards, to verify their subscribers. What does this mean? It means that you can only switch phones with your carrier's permission. Not only that, but the carrier doesn't have to accept any particular phone onto its network. Not that it can't. It could, but typically they choose not to.
Now, some Sprint and Verizon phones do actually now have SIM cards, but that isn't because of CDMA. The SIM cards are generally there for their 4G LTE networks. That is because the LTE standard also uses SIM cards. The phones may also have SIM slots to support foreign GSM networks as in "world phones." That still doesn't leave you the option to switch phones, as those carriers still use CDMA to authenticate their phones on their own home networks.
3G CDMA networks (known as "EV-DO" or "Evolution Data Optimized") also, in general, cannot make voice calls "and" transmit data at the same time. Which is a bit of a draw back. But, you should note that it is an available option (known as "SV-DO" for "Simultaneous Voice and Data Optimization"), they just choose not to have it on their networks.
On the flip side, "all" 3G GSM networks have simultaneous voice and data. Bonus right? They have it because it's a required part of the spec. (3G GSM is also actually a type of CDMA. I'll explain that later.)
It is actually possible to switch from CDMA to GSM. Some Canadian companies have actually done it. In doing so they get access to the wider variety of off-the-shelf GSM phones. However, Verizon and Sprint well they are more than big enough that they can get custom phones built for them. This makes it so that they don't need to switch 3G technologies when they could instead be building out their 4G networks.
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