When you say you want something that comes as a system, I'm wondering what constitutes a system for you. Both are systems, but the Pentax has more options in the way of lenses, viewfinders, grip, focus lever etc.
As others say, the Pentax is not really designed for handheld work even though the ver.II is quite a bit lighter.
The Mamiya, on the other hand, is perfect for hand held work.
If you just want 6x7 and don't need interchangeable lenses I would concur with TimVance about the Fuji GF670. It is fabulous and incredibly compact and portable.