t the larger hard drive. My wife has something like 20,000 photos on our MBP. Plus videos taken with the phone. Plus music. It starts to add up and pretty soon you don't have any hard drive space left. I think we have 256 or so and it's pretty full.
totally depends on your use though. 256 is probably plenty for most people.
Also, from what I gather, the Air is a great computer.