You have to work pretty hard to go over a 2 gig limit--as in downloading a LOT of videos, constant web surfing, etc.
Will you be able to use wi-fi for at least some of the Pandora use, or is that for when you're riding the bike/driving in the car? WI-fi use doesn't count towards your download "limit" so if you'll be using the device at home a lot, no worries.
The only time I've come CLOSE to 2Gigs was last December, and I was using my iPhone CONSTANTLY at my in-law's (no wi-fi) for connecting to the work VPN, surfing the net, etc.
Go with the 2GB plan, your carrier now HAS to notify you if you come close to the plan maximum, and worst-case scenario, you probably have to pay something like $10 per additional GB. You can always upgrade your plan later if it turns out you don't have enough.
Yesterday the Consumerist website had a story about an idiot who spent a month in Canada, was averaging 160+ text messages, plus a LOT of video downloads (hmmmm, I wonder if they were the pornographic variety). His plan (actually his sister's plan) didn't have international roaming, and despite multiple messages from T-mobile, he just continued on.
End of the month bill? $201,000
Moron racks up $201,000 cell phone bill
(lesson to be learned? Turn off roaming when you're outside the US)