I race on Amalfi much more often, however I enjoy both tracks a lot.
I can't really choose a favorite here.
Aria is much more challenging to pass, really teaches you how to pass a car the right way. You also have to watch out for things that stick out.
Amalfi is much easier to me, but still has two difficult sections, notably the fast downhill 'esses' that leads into the sharp left-hander, and of course the last turn, which is just annoying to me.
Aria really gives me the impression of being in the middle of a busy Italian street with the narrow high walls, and Amalfi is just so scenic.
I like both!