da3dl3us, I found this to be the best deciding factor for me. I just checked out the program What Size? Just to see how much space I was using on my current macbook pro and for what.
Application Support is at 47gb
Apple loops is 32gb
Applications 18gb
Plus easily 11gb of stuff for mail, system files, etc.
So, even if I just wanted to keep my applications uniform across my pending new desktop and this macbook air, I'd have to get the 256, because otherwise I'd have 0 space for files.