definately the macbook. and this is why:
Over the past couple of months my powerbook has been through the wars, first it had coke spilled over it, and then last week, i was carrying it around in a tucano second skin INSIDE another more padded record bag, and after putting the record bag on the floor admittedly a little harder than usual, I later found my powerbook to be incredibly dented, actually crumpled, in one corner. Remember that this was after virtually no impact and through two padded cases.
By contrast. The Macbook in my sig was for some reason placed on top of a shelving unit in my house, around eight and a half feet above the floor. As you may have guessed, someone tried to pick it up but accidently pushed the macbook behind the shelving uit, where it fell the eight and a half feet to the floor. The amazing thing was that it had only minor cosmetic damage and seemed o be working fine.
The other thing that would concern longevity of a laptop would be the quality of its parts, and I doubt that there is much if any difference between both laptops in this field.
From my experience with both aluminium and plastic laptops (powerbook and macbook), I seems that the macbook is far more durable.