I have most of the cameras mentioned; the OM-1, OM-2n, ME-Super, MX, ME, Spotmatic. One more I'd like to add to the discussion is the Minolta XD11. It's about the same size as the MX and OM (the ME-Super is actually just a little smaller than the MX), but has an incredibly reliable meter. It also has Aperture-priority and Shutter-priority. This was the first multi-mode SLR and it even had a program mode of sorts; if, while in shutter-priority, you set the camera in such a way that would result in an improper exposure, the camera will shift the "program" to a setting that will properly expose the scene/subject. It's a fantastic camera, which also shares the basic chassis and shutter with the Leica R5 and a couple of other Leica R-something models. Plus, Minolta's Rokkor lenses are very inexpensive but are incredible performers. I love my Pentax cameras, especially the MX, and I love the OM-bodies, but the XD11 is one of my all-time favorite cameras period. Definitely worth taking a look at.
I think the OP has already chosen a camera.
But I share your opinion that the XD-11 is one of the best small manual-focus SLR's available.
I just received one a few weeks ago from an apug member. It is a beater, with peeling leather and multiple scratches and wear.
However I was very impressed with the quiet and low-vibration shutter.
Of the lenses I have, the earlier MC-rokkor-qf's have a nicer build than the latter mc-rokkor-x's i have. I dont have any MD lenses to compare them to.
The lenses are generally a bit larger than the pentax and olympus cousins. Whether this is good or bad I will leave up to the user.