[tx_dbs_tx]

Everybody is right... GSM networks are not nearly as good as CDMA networks. You have to concider these things:
1. Go with the carrier with best coverage/signal for where you will primarily be using your phone. The phone you choose to buy is of no use to you if you don't get a good signal where you will be using it.
2. Pricing. (If that's a factor for you or not). Sprint will beat AT&T by a long shot and you will get much more for your money.
3. Data speeds with Sprint blow AT&T away! If web browsing on your phone is something you like then stay with Sprint. At&T will be decent but not as good as Sprint
4. choose which is more important to you... the phone or the network???
5. I would never go with a specific carrier just for a specific phone unless you are positive their service is good in your area and the pricing/plans are acceptable to you along with CS etc.
6. Last, I would not use an old 2G iPhone even if it was free. AT&T data on 3G is slow enough so imagine how slow 2G must be. Sprint's Rev A data kicks butt (tethering speeds especially) and it will be getting much beter when they start kicking of WIMAX!