Rixen and Kaul make the mounts used by many bag manufacturers. They are pretty close to Ortleib, I dont know which came first. My carradice bar bag with R&K mounts is 13 years old.
They are quick-release with a button, wont sag when loaded and will stay on over rough downhill tracks and trails. Spares are available.
I think the design is better than arkel and you can switch the bags and baskets from other manufacturers.
For bags, check out R&K, Carradice and Basil.