In my opinion I'd go for the the biggest of everything you can get (SSD, Processor, RAM, here's why.
1. No need to worry about "Will I have enough space for the future"
2. You'll be equiped with a computer you can keep longer that can with-stand future software upgrades without another computer purchase.
3. Better resale value, and more importantly more desireable.
In your situation buying a computer with 128GB's of storage, will leave you with aroung 26 GB's left. You actually need a minimum of 5% to 10% free space on you drive for swap files and software updates, leaving you only about 13 GB's free. Unless you do some house cleaning you won't have much space left over. However if you buy the 256 GB Air you'll have little to warry about.
Never really liked putting data on Thumb or SD drive unless it's for tempory files use or transfers because of the potential problems of data loss with no backups.
BTW, I bit the bullet and got the 13" Macbook Air with 256 SSD and i7 processor.
go for the 256 GB flash storage.
this is the computer i have and i chose this because not having a sufficient amount of space can affect the overall speed of your computer. sure you probably wont use up all of your space but at least you spare yourself the trouble of buying and external harddrive and you dont have to ever worry about how youre doing on free space when downloading new programs. the most space taken up on my computer is my itunes music.and if you ever think youre loading too much on to your coputer, try moving some of it to flash drives instead of buying a huge expensive hard drive that will be noisy, heavy, and problably slower than the flash technology found in the air and flash drives.