The 6x7 negative is indeed nice. I discovered it over 30 years ago with my Mamiya Super Press 23. It is probably not as heavy as the Pentax, but naturally not as light as a 35mm SLR. And it doesn't permit viewing through the lens as an SLR does. You can do it but must use a ground glass. It doesn't have the selection of lenses of the Pentax, and it's getting harder to find the 50mm. It does have interchangable backs (can use 220), extention tubes, and a surprisingly 35mm like ease of use. Again, they are getting hard to find, but it is capable of sheet film use.
It is not an SLR if that is what you wish in 6x7, but is worthy of consideration.