Fast external startup-disk for Macbook Air: SSD1 or Thunderbolt?
My personal recommendation would be not to move the OS to external drive as you would have to carry it around just to make the computer run.

I recommend:


Move all the files that you only need slow access to but are GB-eating to iCloud or similar network location. (For example: iTunes Library, Photos, Movies…)
Get a Thunderbolt drive for applications that need speed. Thunderbolt drives are often faster then the internal SSD, the drawback is they would occupy your Thunderbolt port.


Use the About this Mac and take a look what I am talking about.



Also see http://www.cnet.com/topics/storage/best-hard-drives-and-storage/thunderbolt-drives/