Like several others, I use both 35mm and MF. I appreciated the improved gradation and tonality when I bought a Pentax 67. Most shots were smoother with better gradation and excellent sharpness. The difference was there even at 8 x 10. But the Pentax was too big and heavy, and the mirror would hang up in mid-cycle when the battery died--which was often. I traded it for a Hasselblad, which I still have and use. However, I'm not a big fan of square pictures; I don't like hand holding the Hassy with a prism--too awkward--and while the groundglass screen is often just right, I'd rather have eye level viewing most of the time. I use the Hassy for slides, cropped to a wide-screen aspect ratio. I have the Hasselblad projector.
But when cropped to a rectangular format for B&W prints, the Hasselblad is essentially a 645 camera. And that's only half the film area of 6 X 7. It's not the difference between 6 X 6 and 6 X 7; not a difference of just one centimeter (as I've heard Ernst Wildi say). It's really half or double.
But I wasn't going back to the Pentax. Instead, I bit the bullet and went for the Mamiya 7 II that another forum member was selling. I wasn't enjoying or much using my Hasselblad SWC any more, so I first sold that, then bought the Mamiya. I love shooting with it, and printing the 6 X 7 negatives. Back in business again! The rangefinder is the right way to make 6 X 7 comfortable and convenient!
One way I have found to get an IQ with the Leica that comes close to 6 X 7 is to shoot Ilford Delta 100 Pro with an aspherical lens. It narrows the gap, but tonality is still better with 6 X 7.