It really all depends on how complex your design is.
For a simple frame type construction, aluminium can be a lot easier to use. Aluminium also has the advantage that it is waterproof, whereas with wood you'll have to use a waterproof sealer or a weather-resistant paint, which often increases the cost of using wood quite sharply, despite the cost of the raw wood alone generally being cheaper.
And as mentioned earlier, aluminium cages are generally lighter than wooden ones. This lightness comes at a bit of a price though, because if you use the right wood and construct it correctly, a wooden cage is usually more rigid and stronger (unless you're planning to use welded joins on the aluminium, but that would make it far less simple).
There are instructions out there (
http://www.chameleonnews.com/diycage.html and
http://www.baskinglizards.com/product_info.php?products_id=131 come to mind) for reasonably simple cages that are easy and economical to construct out of wood.
However, I've found that if you're building something quite complex, wood is the way to go, because it is generally easier to shape and join the wood to suit the demands of a complex cage design (For instance, I like to incorporate things like horizontal and diagonal beams on the sides of the cages to allow for the attachment of sturdy climbing branches, and also to mount suspended plant pots).
I also find it easier to incorporate other materials into the cage design (like plastics, for the drainage system) more easily into a wooden cage, again because of the increased ability to shape the wood, and also because of the ability to seal the wood more simply than with metal.
As for looking good, there is no doubt that an aluminium cage is neat, but a well built, well finished (painted) wooden cage can truly be a beautiful piece of furniture, not simply a habitat to be tucked away in a dark room.
I do believe that functionality is paramount to any design, but one of the main reasons most people keep these animals is to watch and observe them, and so I've always believed that the aesthetics of their habitat is also a very important factor.
So in summary, there are pros and cons to both materials. In the end it comes down to the needs or your specific design, costs, and also your willingness and ability to work with the specified material.
Will Hayward and Chris Anderson have been incorporating things like corrugated plastic into their designs, and Zerah posted his wonderful outdoor cage made from PVC piping. The use of plastics (like injection moulded beams, PVC's and engineering grade plastics) is something that I am currently investigating, because plastic combines the strengths of both wood and aluminium (waterproof, light, rigid, easy to cut and shape). The only downside in my investigations so far is the cost.
I'll keep you posted with what I come up with.
