Basically, GSM stands to make a lot of improvements and the phone hardware is a lot better, but the CDMA network here in the US is a lot better than the GSM network. Also, most phones that have CDMA technology can use analog lines, so you can make a call from almost everywhere. GSM uses less battery than CDMA, and has the potential for better voice quality, but the actual network integrity of the CDMA network is a lot better. I would ask your local friends to see who's the most happy with their providers, and go with that one. I personally went with Verizon (CDMA) because of better network reliability and better customer service, but I really wouldn't mind a GSM phone. The other plus is that some GSM phones can be used internationally with no problem.

CDMA also, in many populated areas, can support faster data transfer rates than GSM (1.44mbps as opposed to GSM's 1Mbps), and now EV-DO CDMA (allows streaming video content, etc) is available to Verizon customers in certain metropolitan areas.

Or, you could always get Verizon's new Samsung phone that supports both CDMA and GSM, but that phone's ugly