Okay, here's a summary that should help a little:
The US has three major types of networks: TDMA, CDMA, and GSM. As mentioned before, TDMA is the oldest of the three and is being phased out by the companies that still use them (AT&T and Cingular). CDMA and GSM are 2G, or second generation networks. There are advantages and disadvantages for both.
CDMA networks, which are used by Sprint and Verizon, have received what Sprint and Verizon call an upgrade to 3G, but that's not quite accurate as it is more a 2.5G technology now. You may have seen Sprint's new data "Vision" plans which utilize the upgraded capabilities of the network. Initial reports say that the upgrade has not been significant and that both phones and the network are having problems with data. This however does not affect cellular voice communications.
GSM networks, used by T-mobile/Voicestream, Cingular, and AT&T, is the standard network for most of the world, including Europe, South America, and many parts of Asia. The main advantage to GSM is the SIM card, in that you receive a SIM card that can be used on basically any GSM phone. All your personal data, including your address book, is stored on your SIM card, meaning that you're not stuck with one phone. GSM has received an upgrade similar to that of CDMA networks in GPRS, which assists in making data (not voice) transfers quicker. GPRS's success has been better, as it is a bit more mature now.
No one in the US has 3G technology. True 3G technology means that you get DSL speeds (give or take) with your cell phone, compared with what we get (9.6K - 56K). As the growth of the US cellular market is hampered by the sheer size of the area to cover, our technology will lag behind those of Europe and Asia for at least a while. Case in point: Korea and Japan have the most advanced cellular networks (CDMA2000 and variants) in the world right now...what they do with phones (video teleconferencing and such) is quite amazing.
In terms of reception or call quality, your geographic market is the best indicator of the type of service you will receive. As mentioned before, you should definitely ask around in your area to see who seems to have the best quality. Assuming that you're in Berkeley, T-mobile, Cingular, and AT&T should be providing GSM networks, and Sprint and Verizon should be offering CDMA networks. For now, call quality/reception will be very similar for Cingular and T-mobile as they share cell towers. As for the others, your guess is as good as mine sicne I've only been in SD and LA.
After checking out call quality and reception, your next filter should be for providers who provide good plans. If we look just at plans, Cingular, T-mobile, and AT&T all give around 500-600 anytime minutes and 3000 night/weekend minutes for $40/month. Sprint and Verizon give around 300-350 anytime minutse and 3000 night/weekend minutes for the same price. So, unless the three GSM providers' reception really stinks around there, I'd recommend them over Sprint or Verizon. Phone-wise, AT&T (if GSM is there) has the best phones, with the T68i and such, then Sprint (Samsung A500, A460, etc.), then Cingular and T-mobile (V60i, T68), then Verizon.
Okay, so I was bored...enjoy
Edit: About the T68 series...the T68i is the newer version...they are very similar except that the T68i has a bit more memory and has multimedia messaging capabilities. If you want to buy a T68m or T68i, make sure that reception around your area is very good...the T68 series certainly do not have the best reception of the GSM phones out there, but they make up for with that spiffy color screen and all the features (i.e. bluetooth) that they offer. IMHO, the best GSM phone out there is the Samsung S100...yum.
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