I would prefer 48 V, because it is as high as possible while still being safe and leaving some margin (note that 60 V DC is considered maximum safe voltage).

Note that 48 V is 80% of 60 V. So the margin is 20%.

I am against negotiating the voltage dynamically, because I fear that it makes every device drastically more complex, and therefore drastically more expensive. (and drastically more prone to errors)