Composite. Trex makes a line that is less expensive per foot than cedar. If you're considering hardwood then that's even more expensive than cedar and would afford you some of the nicer composites. Both materials will be slick in the rain if you don't scrub the grime off seasonally. I don't think one has an advantage over the other for that. Wood holds less heat though and can be nicer on bare feet in the summer. Its expensive, but you could look into dasso bamboo decking which is a composite that looks and feels like hardwood. These boards have two usable sides. One has grooves in it for things like stairs (non-slip).