Honestly, I also dont buy the film fanatics who say their old lenses and cameras give a great / unique look that cannot be achieved with digital filters. I used to shoot Polapan and do cross processing, and while Nik and Alien Skin have never matched what I got doing that really, their hundred of others takes on old films look pretty darn good to me... I couldn't tell on many an image with their filters in Photoshop the original was not film.. once they put their spin on it...
Well I'm not a fanatic, I'm just a duck hunter in South Dakota. About eight years ago I got back into shooting some film. In the 1990s I shot a Hassleblad for weddings and then added a 4x5 for stock photos. I bought a Nikon DSLR in 2005 and am currently shooting a D800E with state of art Sigma Art and Nikon's best lenses. So why do I still shoot a lot of film? I'm now mostly shooting 4x5 and 5x7, and am semi-seriously looking at 11x14. (They are huge!) I have three sets of lenses for the large format cameras. I have eight lenses made from 1845 to 1860, three made from 1870 to 1900, six made from 1910 to 1930, and five made in the 1990s (mostly plasmats in Copal shutter.) I shoot very little color (Portra 160), lots of b&w film, and increasingly now shooting glass plates (dry plates.) My goal is to get into wet plate collodion this year when it warms up.
With the above background said, I have some thoughts on your questions. I will almost guarantee that I can tell if a photo was made with a lens from the 1850s and shot on glass vs a digital concoction made with modern lenses. Older lenses used different glass (crown & flint), were uncoated, and have drastically different signatures from modern lenses. That alone gives many clues. Add to this the properties of the emulsion on glass plates. They are orthochromatic--sensitive only to blue light & UV. They don't record the full spectrum. And finally they react to light entirely differently from a digital sensor. With sensors the light comes in, hits it, and most is absorbed. With glass plates, light comes in, passes through the emulsion, passes through the surface of the glass, hits the rear surface of the glass, and at that point 90% passes on through but 10% bounces back and goes through the emulsion a second time from the rear. This creates faint halos. This is why modern film has an anti-halonation coating. I've not yet seen any faked digital shot that can replicate all the above. If you know what a REAL "old time" photo looks like the difference is obvious, and you don't have to be a "fanatic."
Now on to more of your questions. My D800E rivals what I get with 4x5 film as far as technical quality, even drum scanned. As a former Hassleblad shooter I'll say that my D800E produces much cleaner images than any 120 film based camera possibly can. If you are looking at a film camera thinking you're going to get technically better images you are poorly informed and completely on the wrong path. So, why do I shoot film when I just said my D800E + Sigma Art lenses produce cleaner images? It's because I'm bored with the "digital look." I love shooting film (and especially glass plates) because of the aesthetics. I just love the classic look I'm getting! It's much more challenging and I feel more involved in photography--the creative/subjective part. I.E., the "art." An 1847 Voigtlander Petzval shot on glass plate or a 1912 Heliar shot on Ilford FP4+ film gives me such a dramatically different look & --feel-- that I just fell in love! I process my own now and that's giving me even MORE creative control.
So what about the difference between Pentax 67 and Mamiya 7? Optically you'll never see a difference. You are asking the wrong question here. The difference is the Pentax is best as a studio camera and the Mamiya 7 is best as a field camera. The Pentax has more lens choices and takes different backs & accessories. It also weighs a ton. The Mamiya is relatively light and quick. And that's always my advice--match the gear to how you use it! Only looking at the tiny technical differences suggests you are on the wrong path.
I take both my Nikon DSLR and a film camera with me on trips. My film camera choices are one of these three: 1937 Voigtlander Bessa (6x9,) 1954 Rolleiflex (6x6), or 1942 Leica IIIc with 28/35/50/90mm. I'd love to have a Mamiya 7 but really can't justify the cost. And, that camera is really too modern for my taste, especially the look the lenses give. My suggestion is to buy a used Chamonix or Wista 4x5, some FP4+, and lenses 240mm Heliar, 165mm Tessar, and 90mm Dagor--all made before 1940 (uncoated glass,) and see what you get. If you just want something fairly cheap but excellent to try, buy the Fuji 6x9 rangefinder mentioned above and some FP4+.
Kent in SD
1. c.1909 Dagor 100mm, FP4+
2. c.1998 Nikkor 300mm, FP4+
3. c.1858 Derogy Petzval, glass plate