It seems that most people carry a space blanket in their kit. It is hard to keep a space blanket wrapped around you and it's not large enough to cover your whole body. If you want to use a space blanket as water proof shelter material or a reflector, the sleeping bag gives you more material to work with for very little more weight or space. I think that the mylar sleeping bag would be more functional than a space blanket.

I would like to get a Adventure Medical Thermo-Lite Bivvy 2. I know that is a large step up in quality over a mylar sleeping bag.

David Enoch