Cave spiders are only usually in abandoned mine shafts or well caves. I have only seen cave spiders from cave spider spawners but they might be out there in caves.

Ups: they drop the same stuff as spiders.

Downs: there are a lot here. 1. They can fit through 1 block holes. 2. like spiders they don't die in daylight. I think. Since they are always in caves you may never know. 3. They can also climb walls and jump really far. 4. Probably the worst one: they can poison you. Isn't that great.

Cave spiders are also smaller then normal spiders. Thats all here.