You've voiced concerns about lens quality. I doubt that either lens is noticeably better; neither Mamiya nor Konica was a slouch in the lens department. However, one big problem with old folders is lens alignment. The folding struts can lose their rigidity or be bent to a degree not visible to the eye, but enough to throw off even the best lens.
From that standpoint, a newer (and rigid) design to consider might be the Fuji GS645S. It has a fixed 60mm lens, about equivalent to a 40mm on full frame 35mm, and is stunningly sharp. It also has a built-in meter that is accurate enough for color transparency film, and is small and light (strap lugs, too!). If that focal length suits you, it might be a good choice.
Fuji also made the GS645. It has a 75mm lens, but is a folder. I've never had one, but it has a reputation for fragility, and for bellows that have deteriorated over time. The bellows are replaceable, but not cheaply.