If the Canonet has a good fast lens and the rangefinders accurate use that, my camera of choice is a Leica with just a 50mm lens and 200 ISO Fuji film, that's worked extremely well at numerous family weddings as I've been able to shoot in the churcj & at the receptions hand held with no flash.

Use the Mamiya for some formal shots, but remember as immediate family you may need to help with the guests etc so need to be fast working.