Thank you, i hope this helps someone else coming upon this thread in future.
i had assumed there may be some difference in the time taken to heat the water, 2 sounding like it would be quicker than 1, but perhaps 1 sucking more heat out with much lower return temps and permitting the boiler to condense for longer and perhaps then more efficient & potentially quicker?
if it makes anyone feel better no one i've asked over the years has been able to come up with a sensible answer as to why 1 setting should be used over another, including Vaillant. I had hoped the collective wisdom here would have a preference but i guess every one leaves it as defaults for customers.
Reducing tank heat time and improving efficiency are obviously desirable and something you may tune on your home systems but perhaps not bother for customers due to potential issues.