Another axis to divide them along is whether they're designed for quick release grab-n-go from the rack, or intended to stay strapped on. Ortlieb is definitely in the former camp here, not sure about arkel.
Arkel has a very secure quick release mechanism as well (+ waterproof zippers & rain covers). Water will eventually work through the fabric unless you have a rain cover. As mentioned, Ortlieb is a dry bag (you could throw it in a river and it would stay dry).