I've not had any direct experience with either brand. But from what I have learned over the years and particulary been told by the owners of both brands I would get a Camano hands down over a Mainship. I have gotten the impression that there is a major quality difference between the two brands.
Others with direct experience with these brands may dispute this and for valid reasons that I'm not aware of. But based on what I have read and been told by people with experience with both, were we in the market for a new boat in the 30' size, Camano would be on the list and Mainship would not.
Of course, if we're talking used boats a lot has to do with how the boat was treated by previous owners.