First decide what you want the rack to do. There are a few styles:
lowrider which hold panniers low down each side of your front wheel. Great for stability and load carrying ability but poor ground clearance. Most common rack for tourists on diamond frame bikes.
platform looks just like a standard rear rack, but over the front wheel. Great for strapping a tent or something lightweight on top as well as a couple of panniers. Less stable but more ground clearance. Occasionally used by mountain bike tourists.
porteur is a platform rack without pannier mounts, usually much larger than a platform rack. Occasionally extends up to the handlebars. Still mounted to the fork rather than the frame, so weight on these racks has a dramatic affect on stability. Uncommon for tourists, more an urban load carrying rack (often used by postal services).
frame mounted. Usually similar to a porteur rack but attached to the frame. Much more stable but now you get interference with cables and handlebars when you try to turn. On a diamond frame bike these usually attach to the down tube and headset. Rare. The real hard-core end, but usually if you're going to this length you'll build a load-carrying bike instead. David Hembrow sells both frame and fork mounted porteur racks (the frame mount ones are not easy to find outside of Europe).
I suggest looking mostly at low-riders, and I think the Tubus Tara is the one to get if you can afford it. In general, aluminium will be cheaper but not last as long, and racks built using both materials should weigh about the same.
If you want to build your own Alex Wetmore has a useful series of blog posts