Whether you need to use the rails to conduct power would be the determining factor for me as to whether brass rail is acceptable. When brass oxidizes, it forms a difficult-to-see non-conductive film (prior to tarnishing, which is advanced oxidation). This oxidation must be removed from brass rail prior to running trains if the trains are drawing current from the track. Frequent use of the trains also removes the oxidation film. However, brass definitely oxidizes more quickly outside, and especially in a salt-air environment.
If you use battery power, brass rail oxidation is not a problem.
Stainless steel and nickel silver oxides are conductive, so are better materials for outside rail if using track power. However, depending upon geographic conditions, the rail will get dirty when not being actively used, regardless of material. Depending on the accumulation and type of dirt, you may have to clean rails before running no matter what.
Equally important to use of track outside is the UV resistance of the plastic in the ties. Some makes can deteriorate to almost unusable in one summer in the direct sunshine in southern latitudes. Some brands are built with much better UV resistance than others. I have seen specific brand recommendations (I model inside myself) for outside use in various forums - you might try a couple of quick searches.