I believe the type of paint on the sterns is completely irrelevant - it won't slow you down if it's the "wrong" paint, will it? As long as it isn't "heavy", and stays there, go with cheap.
So why go with flat black, why not make a statement?
My boat has one stern green, one stern red - guess which is which?
If you trailer your boat, why make the sterns black? Why not white? or even some highly reflective paint? The more visibility on the highway, the better!!