GSM is a global standard that lets you use any phone just by putting in a sim. CDMA is not widely supported outside of the US. But even worse, CDMA is slow compared to GSM (HSDPA).
>"But even worse, CDMA is slow compared to GSM (HSDPA)"
You can't compare CMDA and HSDPA. GSM and CDMA are 2G/3G technologies that support both voice and data. HSDPA is data only and commonly known as 3.5G. It was a stop gap until 4G. CDMA had EV-DO rev B as its 3.5G although most CDMA carriers skipped it in favor of just rolling out LTE.
Lastly LTE(also a standard) phones all have SIM cards. And with Voice Over LTE the "GSM is superior CDMA" discussion is just plain silly. Case in point Verizon is retiring its CDMA network at the end of this year.[1]
Like pretty much every term that consumers hear in wireless, what people mean when they say CDMA varies. The underlying Code-Division Multiple Access method that the OP refers to is a general principle of operation and not a standard. It is also used by HSDPA and LTE. The specific standard that was mostly a US thing and even then specific carriers is CDMA2000, which is a specific standard like HSDPA.
> The underlying Code-Division Multiple Access method that the OP refers to is a general principle of operation and not a standard. It is also used by HSDPA and LTE
Sure, I have a GSM phone partly because I like that the hardware is less tied to the account. Of course, I also had to buy a nano SIM when I bought a new phone, but whatever.
On the other hand, I don't think this is a particularly popular concern. It certainly doesn't concern a large share of wireless customers in the US (they are using the Verizon...).
GSM is a 2G standard [0]. Modern phones are pretty hard to put into a single bin. They often support 2G (GSM[TDMA]/CDMA/GPRS/EDGE, etc), 3G (UMTS[W-CDMA]/CDMA2000/EDGE/HSPA, etc), and 4G (LTE/OFDMA/WiMax, etc).
A modern US iPhone or Android phone is perfectly capable of moving between every major carrier on every technology (its almost always LTE anyways). Every major US carrier uses SIM cards.
[0] https://en.wikipedia.org/wiki/GSM "The Global System for Mobile Communications (GSM) is a standard developed by the European Telecommunications Standards Institute (ETSI) to describe the protocols for second-generation (2G) digital cellular networks used by mobile devices such as mobile phones"
