There is no doubt that the Thermo-Lite Bivvy sack is better (I often carry one of their double sized ones for myself and my wife tro fit in. Misery loves company), but while much tougher, and probably more effecient, it is also much larger than a couple of space blankets. So, while I don't always carry the bivy, I always have a space blanked tucked into my day pack, and usually in a cargo pocket of my pants...