.....there is no difference, once it goes through full diffusion or bounces....
Thanks boa and kd for answering my question.
Looks like both the Metz and the Quantum are both fine systems with differences mainly in the details rather than to do with the "quality" (evenness, softness) of light they produce, which seems to be not too negatively effected by the size of the Metz 60CT flash head if all the positive reports about it are true -- though have not heard from anyone who has had opportunity to compare both of them (flash) head to (flash) head.
I did read one comment on an ancient forum thread that a photog had complained to a dealer that his 60CT unit produced results with brighter concentration of light in the center of the picture and fall off towards the edges that according to him the dealer had confirmed was a problem with all these units so he swapped to a Quantum system and became a happy chappy as his problem disappeared - but maybe the problem arose because he was shooting the powerful 60CT flash straight at his subjects rather than bouncing or diffusing it; my guess is doing this would be softer with the Quantum that has a built in diffuser so perhaps the problem he mentioned is more more to do with the photographer than the gear (as usual).
Another question I have is how complicated and fiddly is it to deal with the dial on the top of the 60CT head? I understand all the theory but am not sure how easy it will be to navigate the (non-illuminated?) dials in a working situation compared to the Quantum's user friendly LCD?
I am writing in March 2022, to add this historical comparison of the Metz and Qflash strobes:
I started using the Metz 402 in 1983, then upgraded to the Metz 60CT series. I set the Metz units aside in 1993, when I transferred to a Qflash T2. I now use the Qflash Trio, with a T5d-r as remotes. The Trio works with HSS, but not the T5d-r, so I must compromise my aperture setting when using the T5d-r for an outdoor session.
The Metz units are super powerful, BUT . . . the power comes from concentration of the light. The Metz flash head is a small rectangle, which creates a very directed throw of light. There is distinct fall off at the edges, obviously. This is fine for a horizontal (landscape) subject, but is a real concern when photographing a vertical (portrait) subject.
This becomes an issue when using a flash bracket that rotates the camera body while the flash remains upright. The horizontal shape of the light output from the Metz becomes very obvious when the camera is in a vertical position.
I had to use the wide-angle diffuser on the Metz head to correct for this disparity.
The Qflash has a round output that does not affect the light fall off as viewed by the camera.
While the Qflash does not have as much power as the Metz, since it is not nearly as "focused", the overall quality of the light striking my subject is softer and more pleasing.
I have stayed with the Qflash, and the latest digital technology has compensated for the lower power of the Qflash when compared to the Metz.