Not to dodge discussion, but my answer is "both."

I have a fleece blanket/bag (fleece blanket that zips closed into a bag) that I got at WalMart for a few bucks. Rolled as it comes it's a bit bulky for carry, but it readily re-rolls into a tube you can strap over your backpack or choulder. It's decently warm, but not waterproof.

I also have a wool blanket, which is also not waterproof but is warm even when wet.

For "waterproofness" I have a SilTarp.

I do have a fancier mummy bag that's light and packs VERY small - but I can't be in it for more than a few minutes before I start to feel claustrophobic!