Plastic is great, until your laptop falls and the plastic shell shatters. That's the weakness of plastic - it's brittle. I have a ten year old macbook with a dinged aluminium chassis. The structure of the shell is still intact despite a few falls.
That being said, aluminum will suffer from dents and scratches. Every material is a trade off. You're not going to pull a "gotcha" comment, here.
Plastic is great, until your laptop falls and the plastic shell shatters. That's the weakness of plastic - it's brittle. I have a ten year old macbook with a dinged aluminium chassis. The structure of the shell is still intact despite a few falls.