Nah, GPRS preferred... that way, as mentioned before, it probably won't go against your SMS plan if you have unlimited internet.
Yeah, I have unlimited SMS as well as unlimited data with the BlackBerry Minutes & More plan from Tmo so I'm not concerned with sending SMS's via GSM. Have you noticed SMS's are sent/received quicker via GPRS? And, if "GPRS Preferred" uses GSM when GPRS is not available, then I'd might as well use that setting.
Ah, you confirmed my assumption about GPRS Preferred.