My take:
I use Ortlieb Roller panniers on all of my off road travels and some on road travels. They are waterproof, even fording streams and in torrential downpours, they are also tough, as in riding through brambles with no issues. I have used regular Cordura bags for on road travels, but they aren't as water proof and seemed a bit more prone to snagging and abrasion.
I have seen Arkel, they are nice well built panniers, but I subscribe more to the small bag in a large bag system of organization.
Buy from
The Touring Store...
Aaron