The SOHC double walled, if I remember correctly, have an air space between the tubes.
And yes, the thicker the metal the easier on the finish, but shouldn't matter much with ceramic coating, and since I don't currently have any painted exhausts, dunno which paint works best or how long it stays on.
I've seen the Mac that r550 was talking about which had the paint turn to a baby#$%* brown in like a couple months, then she had a shop repaint it and it turned some other ugly color in another couple months. And she doesn't ride it that often, shes on her gs1150 a lot more.