I work for a wireless phone company and can at least explain some of the differences, and then you can decide for yourself.
GSM and CDMA are two different technologies. GSM is really TDMA (Time Division Multiple Access), and CDMA is code division multiple access. I will not go into specifics here but will mention the important differences in a moment.
GSM in the US operates around 1900 Mhz, and CDMA operates in the 800 Mhz band. The higher frequency GSM has considerably less range than CDMA. So for a given area using GSM, you must have more cell sites to cover the area.
GSM has approximately 8 times more customers than CDMA, and the phones can be used in Europe, Asia, and Africa. GSM is a standard whereas CDMA is a technology that has to be paid a royalty to Quallcom its inventor.
GSM voice quality is generally better than CDMA. This is due to the technology differences. CDMA is equal in voice quality during low traffic periods, but degrades as traffic increases, the Darth Vader effect.
GSM battery life is longer because the transmitter is only on while you speak. However CDMA is a full duplex technology which allows you to speak and hear at the same time.
So which is better, depends on where you are at. If you are in a populated area and the carrier has the area saturated with cell sites, GSM is a better technology. If you are in a less populated area where the sites have further distances involved then CDMA is a better choice IMO.