Moss will certainly be reduced, however, I have seen very slight moss growth on a metal roof. The roof was much older and in near perfect growing conditions, however, it is possible.

By comparison to asphalt, you will be miles away.

Here is a good site that goes into everything that is metal roofing: http://www.metalroofing.com/v2/forums/index.cfm?action=mboard.members.ViewCategories

Here is a link that might be useful: Metal Roofing Alliance

I wonder whether it depends on the kind of metal roof, angle, and dampness of the environment?

My parents have a gabled, maybe? (I really don't know the right terminology but that's probably not right), nearly-flat roof that was formerly made with a small gravel on it. That grew an impressive crop of moss. Eventually it had to be replaced and they've used a metal and nary a moss tendril has stayed. How general that is I cannot say. It is in the woods of NH, very damp. The angle of the roof is not steep. It's been many years now and I think if it were going to grow back it would have by now. Unfortunately I do not know the kind of metal, though.

You are exactly right.

If the roof is low slope, holds moisture and organic matter, you have the recipe for moss growth.

Moisture + Food source + less exposure = moss growth.

Whatever level of moss you grow on a metal roof will be 10X more on asphalt.