I've got a MacBook and a MacBook Pro.
The MacBook shows up dirt much worse than the Pro in my experience (the MacBook only ever sat on my desk, wheras my Pro goes to school etc.. with me). The Pro will however show up scratches more. I don't wear a watch when I'm using my Pro to prevent it from being scratched.
Another factor is the heat conductivity properties. Aluminium conducts heat much better than the plastics used in the MB, and as a result the Pros tend to run cooler (internally) although feel very slightly warmer externally. The result is a longer lasting HDD and generally the computer should last better.
The last thing to think about is how the materials absorb shocks etc. The MacBook Pros tend to externally appear to suffer worse when they take a knock etc, but this is actually because the aluminium absorbs the energy of any knocks etc.. and protects the internals, particularly the hard drive. The plastics of the MacBooks don't seem to suffer as badly when they take a knock, but in fact it's just that the plastics aren't as 'flexible' and soft as the MacBook Pro's casing, and as a result, all the energy of any shocks get transferred right to the bits inside.
Ultimately, if you look at it simply, plastic seems better than alu, but in reality, the advantages of alu outweigh plastic if you simply look a little deeper...