I prefer a poncho, but that is because I am more likely to get caught in a sudden downpour and want to stay dry / keep moving, than I am to build a shelter / hole up.

But that is my needs, not necessarily yours. (I've actually got some little packs of a space blanket plus a poncho that Tescos were selling off cheap - small and easy, best of both).

Remember the rule of 3s - shelter is a much higher priority than food