I agree that MO has more "older style people" than the Pen but I was unpleasantly surprised last year when I saw so many unmannered guests (old and young) at the MO. So older style doesn't automatically mean "best mannered".
I stayed at both the Pen (again) and the Met (first time) last week. As a repeat guest to BKK I now prefer the Met with "younger style people with good manners" and a modern design approach over every other hotel there. For a first timer the Pen would still be my favourite though.