Poncho. I like the ability to use it as rain gear or shelter or a blanket depending on what I need. Space blankets are too fragile and can't easily be tied into different shapes/covers.

The AMK heatsheet is a great emergency backup for a poncho, but between the two I'd still go with a poncho.

-Blast

Um, are we talking cheap disposable ponchos? My comments were based on the heavier duty/Mil-Spec type of poncho.