I had the same concern when I finally came full-time to Mac from PC and my preference was the Air. PCs in particular make a big deal of large onboard storage, so I was a bit worried about such 'tiny' storage. But then I started doing the math with my folders and became aware of three things -
1/ How little of my stuff was a 'must have' on my notebook and the stuff I don't access all the time or archive (including larger video files) could be stored on a portable external drive.
2/ How very small my files consisting of lots of documents, pdf, pictures and music actually were.
3/ There's not really a lot of additional software I absolutely need to install to do the things I do with my computer.
- and this confirmed for me that the 128 GB Air would be absolutely fine for my needs.
That said, one of the best investments I made on the software front is an application called Xslimmer (
Xslimmer - Your Mac, on a Diet!). This application, very simply and safely, strips unnecessary languages, code for older OSs and computers and a variety of background stuff from apps - and I regained a significant amount of space during its first run. I run it once a month to keep updates and new software purchases lean and I've never had an issue at all. Its Genie is easy and everything it does is reversible.
I've got a completely workable Air here with everything I need on it (and a few things, like movies, I don't) and still have over 50 GB of free space.