Can I choose emergency poncho?

A space blanket is really a small lightweight tarp IMO. Its utility comes from being cheap and lightweight.

I am more worried about getting rained on than much of anything else. Space blankets and bivys do not make very good ponchos, while a poncho can be readily pressed into the same kind of service as a space blanket, at a slightly higher weight and volume price.

A bivy cannot be readily used as a poncho either, although I suppose one could curl up inside one and wait out a rain shower.