I have had my mogul for over 35 years and it has run on 2 O scale layouts and a 3 rail layout I had with Gargraves track. It is still running just fine. I did have a loose driver once I think and simple put some epoxy on the axle end and it has been fine. And that was over 30 years ago. My layout have all been for operation, so there was a lot of back and forth work. I'm sure mine has seen many more than one hours operation and is still going strong. If you like the looks of it, a little TLC might be needed and it will serve just fine. It is not a museum piece by any means but still OK in my opinion. I modernized mine by removing the box headlight and putttin on generator and electric "ash can" headlight. At first glance it looks like a consolidation. I did find out some years ago that it runs the same as my Sunset H8 so I can double head them.
As for the B6, I have never had part come off that I remember. Mine sees a lot of use during an operating session. Usually the switchers run the whole 3 hours hustling back and forth in the yard making and breaking trains. I also have Williams version. They are models of different prototypes. I think the Williams represents some of the last ones built and has the small cab because they were built with power reverse and did not need the space for the big Johnson bar. In addition to the cab, the cylinders are canted at the opposite angle to the ones on the Gem. It is veyr noticable when they are side by side. The gem cylinders lean with tops/valve in and the Williams lean out. As for size, mine are often side by side, and they seem to be the same to me.
Of course these are opinions.