i went with the 256 because:
-it will be my main 'daily' machine with an external screen when i'm at home for the next 2 years
and for me me the daily requirments where: surfing, e-mail, contacts+calender, a few photographs, storing tvshows/movies in 720p/1080p on the go, 10-15 gb of music in itunes, some little coding + development
-so far on my current old gaming pc i'm using (as replacement for my too small and slow mac mini) i've also a 256 GB harddrive but have also lots of games so i know that without the games i'll have plenty of space left on the Air with the 256
-the biggest chunk will be the dvd rips i'll copy over ... and with a bigger drive i i'll have to do less copying back and forth with external drives
-for me it's also some sort of future proofing to operate my machine more freely and if the situation arise to copy suddenly 40 GB of whatever files that i don't have to free up space again
-fewer occasions to boot up the more electrcity hungry PC to do something HD intensive
-regarding the price i didn't found it _that_ big after i had taken a look at the price difference between 128 and 256 drives, especially after i'm on the edu discount (which also made my I7 upgrade easier to stomach with being 87 euro)
in total with apple-on-campus discount and converted from to $ i paid 1.679$ (without tax ... which of course here is astronomic

) for my 256GB i7 which i found "OK" considering Apple historically screwing europe over