Originally Posted by JaccoW

It depends. I think most modern bikes use either plastic SKS fenders or aluminum/plastic laminates by Curana.

However, if long term durability is really important bikes tend to have stainless steel mudguards. I have a couple of vintage mudguards and they all tend to be SS, probably a survivor bias thing. Even modern "OV-fiets" public transport bikes seem to use painted stainless steel models.

I think you see less plain aluminum fenders because most people live quite close to the sea here and leave their bikes out overnight. Meaning bare aluminum tends to rot away in those conditions. So again, it's either painted or just not used at all.
Personally I have bad experiences with aluminum on bikes. On my Gazelle I have had to replace the aluminum kickstand a couple of times already as they just disintegrate after a couple of winters. Pletscher seems to hold up much better.

For this bike I plan on using Gilles Berthoud fenders. They are cheaper around here than Velo Orange aluminum fenders and don't seem to be that much heavier.

Nothing wrong with modern SKS fenders though.