I have the Ortlieb as well as Arkel and the older Carradice system. Here are my experiences.
Ortlieb: modular, easy to adjust fittings, easy to replace parts if you had them with you. I do not like the plastic top rail, which has tended to warp or curve over time. Sometimes they can look clipped on but the lower portion of the clamps is not closed. Bags themselves (Classics) are great, simple and totall waterproof.
Arkel: wish that the bags were waterproof but the mounts are very solid. There is a new version as of last year that is easier to get on and off. Like the solid aluminum top rail. Easy to adjust. Easy to clip on. The locking cams interfere with some type of racks, so try before you buy. The bungee cord keeps them on the rack even if cam is not totally closed. Arkel construction (stitching, materials, zippers) is really great as is customer services. Just wish all the bags were waterproof.
Carradice (older system, before R&K): aluminum top rail, easy to adjust, clip on very securely on the top. Bottom is similar to Ortlieb to adjust. Canvas bags are holding up great and are waterproof to this point. Easy to clip on. Works better on larger diameter rack tubes. Nice retro look, should be easy to repair.
If the Ortlieb QL system had an aluminum rack, it would be perfect. Otherwise, heavy loads could use a little more structure.