Why Bring one? If you get lost, these could save you from freezing to death. They are thin foil-like mylar sheets that you can wrap around you to reflect infrared light (heat) back into the body.

What kind should I get? Some tarps have space blankets built in. It's just for convenience. They don't really weigh much and the do not take up much room. So I can see no other reason to mix a space blanket with a tarp. Just buy any one. They are at all outdoor stores and are even at some drug stores and cost about a quarter (okay, maybe a collar).