What you can see on some houses at the edge of the windowsills. Where water has practically run along the left and right sides and has "etched" itself into the house wall.
Personally, I find granite significantly more valuable, but like many things, it is certainly a matter of taste :-) With aluminum, I could imagine that rain sounds are more noticeable, but that is just a guess.
Putting windows on the insulation layer doesn't work.
Doesn't work is not an option. Just google "Installation of building components in the insulation layer" or "Windows in the insulation layer". But beware, it requires careful and conscientious planning and execution! So ask to see references and only hire contractors who have done this before ...
In the meantime, it has already rained quite a bit on our aluminum window sills. I haven't heard anything. Maybe it's because of the triple glazing ... ;-).
You should choose the window sill that you like best aesthetically. Or go by the price ... I consider the type of exterior window sills to be one of the rather unimportant things in a single-family house. It's six of one, half a dozen of the other ... The main thing is that it is carried out professionally and without defects, especially concerning the sealing and the connection to the facade.