Prep is the answer. You may see rib lines that you absolutely can't see when white. You really need to fair it out with a long board before you even prime. Good luck. Sounds like a really hard job.
Is there any technique to determine if the sides do have undulations due to ribs? I agree, it would be difficult to see imperfections in a light color.
I think at this point, I am leaning a little more toward using Perfection rather than Brightsides. I assume neither one is really very repairable. I had trouble last summer with painting Perfection on the decks, in that, it dried really dull when in direct sunlight. I have to travel to the boat, so, can't pick perfect days. However, the topsides will not have as direct sunlight.