My Mamiya 645 Pro shoots horizontals when held normally, FWIW. It's easy enough to turn for verticals, at least with the winder grip which I also have. It's certainly big compared to my 35mm Pentaxes, but it handles easily and the lenses are superb. Can't compare them to 50 1.5 Sonnar though, as I've no experience with one of those.
Well, *that* was an enabling post. I just pulled the trigger on an M645 outfit from KEH. Didn't realize how inexpensive they'd gotten! It defaults to vertical, but I'll manage.
San Diego, CA, USA
The lady of the house has to be a pretty swell sort of person to put up with the annoyance of a photographer.
-The Little Technical Library, _Developing, Printing, And Enlarging_