Personally, I don't understand aluminum chassis. Sure, it feels more premium, but it comes with quite a bit more weight than plastic, and I much prefer less weight over "feel" when it comes to a laptop.

Plastic. Never again will I allow a laptop to be unusable after some time because of a cracked chassis that couldn't handle mechanical and thermal stress correctly. Never.

It manages heat better, with the entire chassis acting as a heatsink. Also unlike plastic, under repetetive heat/cool cycles the tolerances won't change like plastic will.