Can you please clarify your question?
If your question is which one should you use, you need to determine how you are to use them for your pre-paid.
The AXT only has FXO ports which you would use to connect to phone lines to send calls out to or receive calls in from (typical pre-paid is to receive calls from phone line and send over IP).
The AXG only has FXS or station ports that you would normally connect to phones or to the trunk card of a PBX. You could use this in a call shop where phones are connected to the FXS ports.
Of course you will need an radius server to handle all of the databasing for your calling card operation.