Actually, I was applying a general principle in a post about "normal" modems, but your're right; GPRS is far worse - you've got all the "normal" modem worries, plus the vagaries of a radio link plus an IP network!!

The only (and major) actual profit is that you pay "per byte". So if your device is to send i.e. 100 bytes every half a hour (and never expect "incoming calls", all the commands received within "communication timeframe" GPRS would be the way to go. You avoid spam by staying online only 20s or so every hour (you don't keep your connection alive all the time), you don't depend on the 4s connections (if your devices heavily abused them, the provider would certainly end supporting them) and i.e. you simplify the error recovery procedure: Dial, send your data, expect certain ACK (i.e. checksum) reply, containing any additional data/commands, hang up. If no ACK is received, hang up, repeat. If within n (say 10) retries you don't get the ACK, send SMS to service (normal human) phone. Retry every x minutes (say, 5) until normal operation is returned, then return to your 30min timeframe. Same about the server side, if the device hasn't called within a hour, notify the admin.

Connection error recovery could be a whole domain of science. There's so many things that can go wrong that you can't foresee many of them. Even if you've foreseen what just happened, you often lack tools to detect this particular failure. Even if you detect an error, you often can't diagnose its reason. Even if you can diagnose it correctly, usually you can't fix it. Often the error prevents you from "calling home for help" as well. So in great most cases your recovery options are very limited, which simplifies the problem - instead of 2000 different error recovery routines, have 3-4 simple routines that do what can be done depending on how serious the error is.
1) Nothing serious. Go on with normal routine, just notify the service that it happened.
2) Something important. Issue a serious warning. Keep retrying if necessary.
3) Critical. Back up your files and shut down before something worse happens.
4) Specific. Something happened you can attempt to fix. Try to fix it.