Delrin (1/4") is the lightest and slides the best over rocks, logs, and other trail debris. Brass (1/4") is the heavy weight, typically used as lower links for good vehicle balance. It typically slides better then aluminum.
Why don't we do aluminum? Aluminum tends to grab at rocks and stick to them. It also bends fairly easily, for this reason we don't do aluminum links. If you are wanting light, Delrin will slide better and hold up better then aluminum and is super light.