I'm sure that some experts will chime in, but the ribcase gearbox is far superior. The main difference seems to be the synchros. The smoothcase synchros are "shrunk" onto the gears and require additional machining afterwards. No one seems to have the expertise to do this anymore but it's a non-issue since these synchros haven't been available for decades. Oh, and those old synchros didn't last very long. The ribcase gearboxes are very sturdy and seem to hold up very well even in racing situations. I think the ribcase also has better gear ratios. For people looking to maintain original appearances, I've read that it's possible to put ribcase guts into smoothcase housings with a little machining.