Gotta watch price/minute AND expiration dates. I would suggest Boost Mobile because their minutes don't expire for 90 days. So if you won't be using it that much you could conceivably make a $20 dollar card last 3 months. Service is good, but texting is a little slow on their network. (It might take a minute or two to send/receive texts--just every now and then...on average just a little slower than most phones)...oh every minute or text is 10 cents. They have $20, $30 and $50 cards each with 90 day expiration (the $50 might have 120)
If you think you might end up using up more minutes then I would go with Virgin. (^funny guys). They currently have the most bang for the buck. It's a prepaid monthly plan (after 30 days service is cut off, no rollover minutes I think) but it's only $30 for around 1500 minutes. No one comes close to that 1500 for $30 as far as I know. You might have to buy online for this deal, because I did not see it when I was out recently shopping but it's called PayLo service, maybe you could find it in your neighborhood.
Watch out for those Wal-mart Straight-Talk ones. The cards say "up to 1500" for like $20 but that's B.S. They round DOWN, so that's good, but basically it's a 750 minute card...and to get 1500 minutes you have to take advantage of the rounding down....by hanging up on people/keeping your calls, EVERY SINGLE CALL under 45 seconds or something. Do-able, but difficult.