honestly AT&T does not have the best 3g network... Verizon does, and TBH Verizon has the best coverage and they seem to have reasonable rate plans.
The ONLY think AT&T has going for it is they have the iPhone and even now they are losing ground on that because of Verizon and the Droid... i just ordered 2 Droids and they should arrive tomarrow, they cost 50 bucks at newegg.. i got a family plan which is 700 minutes a month (we never use that much) unlimited to all other Verizon phones unlimited nights and weekends. On the Phone that does text i spent 5 bucks more a month for 250 texts a month so the calling plan is 74 bucks a month
the drawback is that with the "smart phones" you have to have one of their internet packages to use them and they are 30 bucks a month per phone
but still all in all i got 2 Droids for 100 bucks total then a monthly bill of about 150-160 a month and that also has unlimited internet on the phones
Verizon's 3G network has more coverage, but certainly NOT more speed
AT&T wins hands down on speed
NOW as far as the OP's question
it is pretty rare to find a dual mode phone that runs on both CDMA and GSM
Theoretically any GSM phone can run on any GSM network, and any CDMA phone can run on any CDMA network...given the correct unlocks and settings
GSM Companies would include:
T-Mobile
AT&T
Voda-Phone
Rogers (Canada)
CDMA Companies would include:
Cricket
Verizon
Nextel/Sprint
Boost Mobile
Go-Phone
CDMA works on an OTA authentication system and does not require the SIM card like a GSM tower would...if you want dual service then you are probably better off with a GSM service
If you can't find a dual sim GSM phone, then simply unlock (or purchase an unlocked phone) and swap sims out when you feel the need to use the other carrier
PS. T-Mobile has decent 3G coverage and is by far the cheapest...however T-Mobile's 3G runs on the 1700/2100 band, where as the AT&T runs on the 1900/2100 UMTS band... you won't find a phone that has dual compatibility and will end up with edge on one of those networks
If this service from Mexico runs on the 1900/2100 band then go with AT&T... if it runs on the 1700/2100 band...go with T-Mobile (Though I believe T-mobiles frequencies are unique)