I have used the emergency bivy bag, it's aluminized tyvek i think, quite good and more durable than emergency blankets especially the mylar ones. I have, but have not tried using the heat sheet from AMK. I just replaced my old mylar ones. If durability and weight are not an issue, i would go for the biv bag, if you can't spare the space or weight the heat sheet would be a better choice. Ideally both, you could then use the heatsheet as a tarp and would be very visible from the air.