I don't believe there's a standard definition for 'university student'.
How much space are you using on your other computers?
I probably should of been more specific on that one
On my iMac I have most of my videos, all of my music and so on, with the 3TB Fusion drive I have around 2.38TB left. On my 2011 MacBook Pro I have 463.17GB free out of 749.3
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What are you using the macbook for? Apps? Files?
In general 256gb is plenty as long as you dont do a lot of media work.
I have 256gb on my rmbp and I have a 2tb external for my photo libary.
Withou the photo libary the 256gb is enough.
Itunes files (movies, music) I keep on my mini.
I mainly use my MacBook Pro for writing documents in Pages, watching iTunes content (mainly TV shows) but these are in iTunes in the cloud. I also do a lot of web browsing and work with Final Draft, iBooks Author and just general writing.
The main reasons I've been thinking about a MacBook Air is because of portability and the brilliant 12 hour battery life I have been hearing about. Don't get me wrong I love my 15" MacBook Pro 2011, it still works great apart from a little slow on boot up, but it's not very practical to carry around in my bag when commuting to and from university on the bus, especially with other things in my bag. Also the SSD speeds would probably be more help.