Supercharger=more air
Nitrous=more air + nitrogen. The nitrogen helps to prevent detonation, where as the supercharger has nothing, unless it has an intercooler which doesn't help as much. The nitrous also cools the charge alot which helps to prevent detonation. That's one reason why you can run higher compression on nitrous engines.
as much as you want. I was reading where billy glidden runs 15-16:1 compression spraying the crap out of it. Compression just makes the engine more efficient until you get to high and reach a point of diminishing returns (heat becomes the enemy, creating detonation), then you can just take the spark plugs out and run diesel fuel in it!