fuse or breaker?

I can't tell you what  you should fuse the amp at, but I can tell you that a circuit breaker will only be trouble down the road.  An ANL fuse would be a much better choice.  Breakers depend on heat to determine how much current is going through the device.  Engine heat added to the heat of the contacts usually shorten the life of the breaker.  Does the amp have fuses mounted to the end of it?  If so, the fuses in the amp should protect the amp.  The fuse under the hood is there to protect the wire going to the amp.  An 8 Ga. wire has a safe current handling capacity of 100 amps.  A 4 Ga. wire is capable of carrying 150 amps.   2 Ga. = 225 amps.  If the amp has no fuses mounted on it, you will need to do some more research to find out what it needs to be fused at.

A lot of people on here will recomend fusing the wire with the maximum size fuse that the wire can handle.  However, it is much safer to fuse the wire for the maximum current YOUR system requires.  Every fuse  has a rated time delay before it blows - the bigger the fuse, the longer the time needed above the rated current before it will open up.  If you are running a 4 awg wire but your amp only has (3) 30A fuses in the side of it there is no point to use a 150A fuse when 90A is all you are going to pull. 

My point is, if you are only going to pull X amps, you don't need an X + 50 amp fuse protecting the wire.  The bigger the current the more important this becomes.