The Airstash looks like the more flexible and portable solution, good for short trips. The GoFlex Satellite looks like something I'd like to have if I needed to carry my entire media library around with me, and then some.
So, I guess it depends on how you intend to use it, and price and availability of course.
So I just want to clarify that an SD card is an SD card, right? The airstash doesn't take a special kind of SD card, right?