mobius - I also tried it back in the day when I had a Profitec 700 and adjusting the pump pressure was a lot easier than the Elizabeth :-)
At the time, I remember writing on CFUK that I preferred 9bar shots as far as an espresso pulled at a ratio of 1:2 is concerned. I think the 6bar shots are favoured for longer ratios and light roasts, as well as the
gushers turbo shots.
Unless you drink lots of espresso on a daily basis, I think it will be hard to be able to judge which one you prefer, unless you do a side by side experiment.
Try pulling shots at 6 bar for 3 days and stick with it. And then go back to 9 bar for another 3 days and then back to 6 bar. Ideally with the same coffee. Let you know which one you prefer.