Given how quickly SSD prices are falling, now that the MacBook Airs have USB 3.0 ports, it might just be better to purchase a cheap USB 3.0 external enclosure and a higher capacity SSD when the time comes that you need the extra storage.
I agree with KPOM that SSD prices keep falling, and will most likely continue into next year. So if this is the case, my considerations are:
1) If you need the extra 255gb of storage right now and will need it for the next couple of years, then get the 512.
2) If you are just worried about not having enough storage in the future without any specific needs right now, then wait. Yea, future proofing 3-5 years down the road is good, but I bet the price options for the next 2 generations of Airs will have the 512gb option at a lot cheaper price then now. If that's the case, you might just want to take the $500 you save now and upgrade to a better machine for $1000 or $1500 that it would take to get a new one.
3) Resale on the 512gb will be worse percentage-wise then the 128 or 256. Basically people don't care that you paid $500 extra new when you try to sell the machine. They usually want the cheapest one they can get