Hi T–1000.
I think that for the workflow that I employ, particularly using my Imacon 949 and the resolutions it scans at for 6x7cm vs. 4x5", I think the quality is essentially the same. I'm sure if I owned a drum scanner and was as good at operating it as the guys that write for On Landscape (Tim Parkin, etc,) then I'd see some benefit in shooting 4x5" in the detail and tonality department. Optical printing would no doubt further emphasise the difference. Trouble is, shooting sheet film introduces a whole lot of other problems. For one it's hard to keep film flat in the holders, especially in variable temperatures. Also, wind vibrations etc. are a lot harder to manage with a bigger camera. And I won't forget to mention the cost to process sheet film where I live is insane, and the results inconsistent at best...
On my Techno, I shoot using a 55mm Rodenstock APO-Sironar Digital and 90mm Rodenstock HR Digaron–W, mostly at f11 or f16. On my Technika, I use a 90mm MC f8 Super–Angulon and a 150mm Nikkor–W. The 90mm isn't the sharpest lens ever made, but the Nikkor is fine. I shoot them mostly at f22, very occasionally wide open. The digital lenses are markedly better when examining fine details of negatives. To freeze motion, I usually have to use 400 Portra with the 4x5", but can shoot 160 Portra when shooting 6x7cm.
The other advantage of the Techno is that the movements are a heck of a lot more flexible and the gears (my Technika has none, bar the ratcheted front rise,) far more accurate. It's FAR easier to dial in appropriate tilt etc to get the desired DOF. Also, the GG is far finer, brighter and more contrasty than the one on my Technika – which is a current series GG and fresnel made by Linhof and bought brand new to replace the original, which was badly scratched when I bought the camera. I use a 12x loupe from Linhof Studio.
All in all, buying the Techno was one of the best decisions I ever made. I feel like I've lost nothing in the way of quality moving down formats, and gained a lot in the way of movements and general pleasure when using the camera. (Note though that I only shoot colour negative, mostly at the speeds mentioned above.)
Looking forward to bolting a digital back on the beast soon.