Good wool blankets are heavy, expensive, and thoroughly wonderful.

But for family kits or car kits, I would probably go with fleece:

- weight: fleece wins hands down; a kit that's too heavy tends to be left at home
- cost: you can afford to double the number of blankets you include (though to compare apples to apples you have to account for size differences)
- disposability: you can afford to lose a fleece blanket to cover, say, an accident victim; and you won't hesistate to cut it up to improvise socks, mitts, toques, etc.
- washability: much easier to wash, bleach, and dry fleece away from home; especially useful when dealing with kids or illness.

The only drawback is size -- cheap fleeces are often short. I often end up using two. An uncle of mine buys better-quality fleece by the yard from a fabric shop. That way you can make the blankets as long as you want.