new ultegra r8000 rotors? or older xtr/saint rt99?

I can find them at various places for roughly the same price

I don’t know if one is supposed to be better than the other, but I think the ultegra ones look pretty ugly

Ultergra, designed for the road and is quite small and tucked away so won’t be too ugly.

how much smaller can one rotor be than another

The road specific ones will cool better due to the scalloped vents in the centre spider. I went for Hope as I needed 6 bolt.

All will stop you adequately. Therefore lightest weight wins.

I run Hopes on both roadies. Lightweights on the lighter one.