Disclosure: I am not an outdoor G modeler - my recommendations are based on extensive reading, and inside use of brass, steel, nickel silver, and stainless steel rail (mosty in HO and 3 rail O gauge).
http://www.btcomm.com/trains/special_offers/track/track.htm#materials is a web site discussing various types of G track.
Brass rail is used by Aristo-Craft and LGB, both of which have UV-stabilized ties. The mass of G scale trains, especially with metal wheels, does a pretty good job of breaking down brass oxide at the inside corner of the rails (where the electrical connection is made). Brass oxide is a non-conductive film that occurs naturally (eventually becomes tarnish if left alone). However, getting that first train to run consistently in the Spring, or after a week of non-use in your area usually means having to clean the track manually to remove the oxide. Wahl's Clipper oil is used by some on indoor track to retard the formation of brass oxide. There used to be a product in the '50s and '60s called No-Ox that did the same thing, but I haven't heard of it in years. Brass oxide is very slow at breaking down the material itself; the surface oxidation forms a protective barrier that makes it hard for the oxidation to penetrate deep into the brass. Except for the oxidation problem, and the gold color when the running surface is clean, brass rail is probably the best all-around.
The oxide of steel is rust - also non-conductive. The big problem with rust if left in place is that it generally expands the metal with fastening-breaking force over time, and penetrates deep into the metal by continually exposing new layers of metal to the air. Rust can be inhibited with use of materials such as WD-40, or plating with tin. The use of WD-40 or other oil films on the track may cause dirt to accumulate on the wheels. Overall, I see the only advantage of steel rail is that it looks like steel rail. I think it would be more maintenance-intensive than brass in your humid and wet climate. Once the rail base starts to rust, the track will be difficult to keep smooth, and will likely have to be replaced.
Stainless steel appears to have a 25-30% cost premium compared to brass, but appears to be the most maintenace-free rail material. Stainless steel surface oxide is clear, protects the metal underneath, and conducts moderately well. The only drawbacks I see to stainless steel are cost, having to paint the outside and lower surfaces of the rail for realism, and depending on size of railway, may need track feeders more frequently than brass. Soldering stainless steel is difficult. It is my personal choice in G, because it is cheaper than nickel silver.
The oxide of nickel silver is clear, and moderately conductive just like stainless steel. However, it is the most expensive track and doesn't appear to have the wide range of sizes that brass and stainless steel do. Conducts almost as well as clean brass, and solders as well as brass. If cost was no object, this would be my choice of rail material. It is the material of choice in smaller scales indoors.
I would not touch aluminum rail outdoors personally. Although clean aluminum has the highest conductivity of the rail materials, the oxide is an ugly color, powdery, and non-conductive. Aluminum oxidizes very easily, and the oxidation destroys the base metal like rust to iron and steel. Because of this, most aluminum is anodized, painted, or alloyed to increase corrosion resistance. Only a relatively non-corrosive alloy should be used. Soldering is very difficult, and making electrical connections to aluminum is usually a point of corrosion entry unless a weather barrier is applied afterwards. Aluminum track is not as resistant to damage from being stepped on as are the other materials. Aluminum track appears to be the cheapest.
The brands I have seen most recommended for outside track are LGB and Aristo-Craft. Aristo-Craft has both stainless steel and brass rail track, LGB has brass.
Hope this helps.