The MacBook is built from polycarbonate (plastic like) material.
The MacBook Air is built from aluminum.The Air is silver in color throughout, but has a black keyboard.
Both the MacBook and the Air are constructed using similar unibody construction. The aluminum Air is made of a stronger material. However, the Air is also much thinner, which could also be a downplay to its durability. The polycarbonate MacBook is a better material for Wifi signals. While the aluminum used on the Air is stronger, this can sometimes be to its deteriment, as the material will dent.
With normal use, the MacBook's finish is prone to "surface scratches." However, they are not as noticeable unless viewed at the right angle because of the white finish. With normal use, the MacBook Air's aluminum finish can be scratched, but it is a different kind of scratching than with the MacBook. When a MacBook Air is scratched the damage tends to be more noticeable.
The MacBook has had some reports of the rubber bottom piece not being very durable, and Apple has issued a plan to replace this component should it fail. The white finish on the MacBook can stain, particularly the palmrest area. The aluminum finish on the Air is more resistant to stains from hand oils and other toxins. The keyboards on both notebooks, black (Air) and white (MacBook), will become shiny and wear slightly with use.
If WiFi signal strength is important, the MacBook's polycarbonate material makes it the better choice. The overall durability of both laptops is about equal , although the aluminum Air has a slight edge.