We've done both, and prefer to buy once we get to the hotel. We use our DC, which I think gives us about the same discount as AAA would. We used to always purchase prior to leaving for WDW. But on one of my sister's trips, my BIL forgot their passes at home and didn't realize it til they got to Virginia. They got to DW, and explained the problem to a CM, who pretty much said, tough for you. He asked to speak with her supervisor, and at that point they did something with the computer, replaced the passes, and the situation was salvaged. So now, just in case we too suffer a 'senior moment', we wait til we get there!!!