I'd take the blanket.

The blanket is designed to keep in more heat. The purpose of both is to keep you warm and/or dry. Blanket should keep you warmer, and with uses drier.

My biggest issue with taking it would be it's durability.
I think the tendency to delaminate reduces a space blankets durability, but the difference in durability I don't think is that long so as to change to the poncho.