If you have the option to switch to an Nexus with ICS, move to CDMA, and keep the $35 price tag then that is what you do. You coverage will be better, your data speeds will be better, and the phone far superior in all aspects but one.(security)
The absolute cheapest smartphone plan with AT&T is $60/month. But that is only going to give you 200MB of data and no texts. Move to 3GB's of data and unlimited texts is going to add $30/month.
If you are generally happy with your coverage but want to move to post paid then look at Sprint. $80/month 450 peak minutes and unlimited everything else. If your lucky and live in a Sprint LTE area it's a no brainer if you want post paid to go with them.