I think the 2 basic choices are a one big drybag thing vs 80 gazillion little compartments and pockets. You would find ortlieb pretty much at the top of the price heap for the former, and arkel for the latter, and then price down from there.

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. I think both ortlieb and arkel sell their mounting systems to retrofit onto other panniers.