Print quality of 6x7 film vs. 35mm digital images is no simple topic to discuss. Actually it's a mine field with countless variables. This topic has been discussed in this forum a number of times and 20 people will have 30 different opinions, depending upon the variables. This is a hot potato I wouldn't want to write about. I know what's sufficient for my needs, but others will surely have differing opinion. For me, scans with a flat bead Epson V850 are good enough for me to print to A2. I also print my Canon 5D Mk III and Fujifilm APS size images that large. Since I don't print on glossy papers, nobody has ever mentioned a quality difference. Obviously I know how each image was taken and processed, so I'm not really an objective critic.
I never noticed a quality difference between my Pentax images and those from my neighbor with his Mamiya 6. So I imagine that the lenses are of equally high quality. There's more to a technically "good" image that just hardware - a sturdy stance or tripod is necessary, a proper shutter speed, proper focus, aperture, exposure, etc. If I were to chose a system, I wouldn't worry about lens quality. I'd care more about handling (weight), build-in lens shades, available lenses (wide angle, macro, ...) etc., but that's my personal view.
Leaf shutter vs. focal plane. You can google the types for a better explanation than I can give. Bottom line when shooting with flash: cameras (or lenses) with a leaf shutter can be used at any shutter speed, where cameras or that era had a flash sync speed of 1/30 or 1/60 as quickest shutter speed.
I don't know which focal length was the wides for the Mamiya 6 and 7. I recall there being a "very wide" lens which used a viewfinder placed onto the hot shoe, but I don't remember details. Lens requirements will dictate which system is right for you. My wides lens was the excellent 45mm for the Pentax.
The Pentax looks and operates like an oversized Spotmatic or K1000, but I was never proficient hand holding it. Yes, I guess some people were happy shooting hand held, I preferred mounting it on a tripod. Today I regularly shoot my Fujifilm X100T hand held a 1/4 second with decent results - other people wouldn't consider a camera without image stabilizing. Different strokes for different folks. Sometimes it seems a miracle that people were able to take decent photos without today's bells & whistles!
Medium format film systems are still a considerable investment today. Make sure you really want to travel that route before taking the step. Finally, in my view, your decision should also be based on your usage and final output. If you print "large", then medium format may make sense, but what are you missing with your current digital gear? I'm nostalgic, I admit, but I've learned to embrace digital cameras and no longer use my analog gear, which is simply too tedious (and costly).