Assuming you're talking about a pizza peel for home use, wood is better than aluminum. Here's why:
Wood peels won't stick to your pizza dough as much as aluminum peels will. This means less frustration when trying to get your pizzas onto and off of the peel.
Wood peels are also more forgiving if you happen to drop them (on accident, of course). Aluminum peels can dent or bend easily, making them unusable.
Finally, wood just looks nicer than aluminum! Whether you're using your peel for display purposes or not, it's always nice to have a nice-looking kitchen gadget.