We use both Ortliebs and Arkels when touring on our tandem. We have Arkel TT-84 tandem panniers on the rear and Arkel back-roller pluses on the front. I like both brands. The Ortliebs weigh significantly less since they are smaller and the Arkel tandem panniers have an aluminum back panel in them. I like the multiple pockets on the Arkels. It is much easier to find things in the panniers than looking in one large compartment. The Arkels are very well made, and Arkel provides great service. The hook on the pannier bent after we had the panniers a number of years, and Arkel sent us a new, improved one for free. The map case on our Arkel handlebar bag started to come apart at the seam after a number of years, and Arkel sent us a free one that made much better. The inside of the Arkels will stay dry in a light rain without putting the rain cover on. Putting items that you do not want to get wet in plastic bags will always keep them dry in the Arkels.

The Ortliebs are light and simple. They keep everything dry. Of course damp items will not dry inside the Ortliebs, while they may get somewhat drier inside the Arkels. The lining in my first set of back-roller pluses eventually wore out, but we got many years of use from them before that happened including using them for many shopping trips when we were not touring. Sharp objects can damage the lining in the back-roller pluses, but this in not an issue if you use reasonable care.

We generally put our clothes, prescriptions, and other items that we definitely do not want to get wet, and which we will not need during the day, in the Ortilebs. We always put stoves, cooking pans and utensils, our tent, and other items that can get wet in the Arkels.

Both companies make great panniers, and you cannot go wrong with either one. The main differences in them for use are that the Ortliebs are waterproof and the Arkels have multiple pockets, which we find very convenient.