I use a Herschel sleeve for my 13 inch MacBook Air. I absolutely love it! I use a sleeve because using a case would defeat the whole purpose of the air being thin and light. That's just my opinion though what do you use to protect your MacBook??
I use an Undercover Secret Sleeve for my MacBook Pro.
http://www.amazon.com/Undercover-Se...ag=hawk-future-20&ascsubtag=UUimUvbUpU2516881