256gb
I have 128GB at the moment, but I would go for 256GB if I were to buy a new one now.
This is my first MBA, but it is not my only machine. Since first getting it two years ago I have found I use it far more than any of my other machines. It is so portable I take it with me most places I go. It is also so powerful and capable it can do everything I ask of a computer and more.
As my usage of this machine has increased so have the number of apps, docs, music, and data I want to have with me. My hard drive is not yet full, but at my current rate of expansion, it could become so very soon.
Go for the 256GB model. But the 512GB one is cavernous for a portable machine.