I went 256GB for the 2013, and had 256 in my last Air. It might get tight if you want to run a big bootcamp partition, or want to edit HD video - but if not you'll be fine. I plan on getting a external HDD for archiving files I dont often use but dont want to delete.
To make simple things short and easy for you, i was also getting 256GB till i realized the reasons of buying a Macbook Air 11'/i5/8GB/512 SSD
1. Ultraportability: Yes that's what we buy this mac air for ==> i am simply not ok with the idea of wires hanging down with this 11' as an external HDD plugging into USB --> NO way
2. Battery Life ---> More empty space = better performance = more room for your files to breathe ==> longer duration satisfaction as you will NOT max out on your SSD quickly as that can so easily happen with a 256GB, for example if you have a DSLR camera and shoot pics, then those files are big in size...
3. Better performance and faster SSD: Yes, believe it or not, many users have reported that standard 128GB SSD is from Sandisk and SLOWER than 256/512GB SSD from SAmsung in 11' Air
4. SSD/Flash is the fastest technology for a mac and i am sure there are all sort of cheap alternatives out there = 90$ 1TB USB3.0 and cheap 160$ 2TB Western Digital and 270$ time-capsule and what not....but they all have their own limitations
YOu cannot upgrade this AIR once you buy it and you know it...so rather burn a bridge now and opt for the 512