I've used both the Arkel TT-84 and Ortlieb 'Bent panniers on my trikes (Greenspeed GTT tandem & ICE XL NT single.)
The TT-84's were purchased for the Greenspeed, and managed to hold everything the 2 of us needed for extended tours (with the excess laid out on the rack). One necessary change was to move the mounting hooks down in order to raise the panniers for more ground clearance given their height & the fact they were being used with a 20" rear wheel. I used them on the ICE for several years and had ground clearance problems on uneven roads; the rear rack on this trike is mounted lower than that on the Greenspeed. Other than this issue, they were typical Arkel - bombproof, lots of pockets, and despite the tours/miles put on them looked almost the same when I sold them as they did the day they were purchased. As for waterproofness, technically they're not, but there were several times we were caught in downpours without the rain covers on & nothing ever got wet. No problems with weight over the rear wheel - remember that a 20" wheel is inherently stronger than a larger one.
I decided to replace them solely because they had too much capacity for me when touring alone - it became one of those "Gee, I've got some empty room so I might as well carry some more equipment" :-} scenarios.
Checked out both the Arkel RT-60's & Ortliebs - I too had no dealer even remotely close and decided to order both - they're so different I saw no other choice (the Arkel vs. Ortlieb design differences have been discussed/debated ad nauseum). In the end I decided on the Ortliebs because MY gear fit the one compartment design the best. I have not considered Ortlieb's in the past because I don't care for a tall, single compartment design that I have to dig down into; their 'bent panniers offer the option of one relatively shallow compartment.
The positive - they're huge, seemingly much larger than their capacity might indicate. I'm able to get all my gear in the 2 panniers - nothing on the rear rack - and I'm what I would probably consider a middle-of-the-road load carrier.
The negatives to me are numerous, 'tho I hasten to add they are not deal-breakers so much as design features that could have easily been dealt with. The panniers were designed, according to Ortlieb, to fit "the 5 most popular recumbents sold in Europe" meaning 'bents with racks that tend to be longer than those commonly found here (such as the modified Tubus Cargo on my ICE). In addition, the bar that mounts the hooks is one piece but divided into 2 sections. If Murphy treats you as s/he does me, the perfect spot for the rear hook will fall in the unusable space between the 2 sections (an Ortlieb rep told me to not even bother trying them as they wouldn't fit my rack). If the rack were 1/2" shorter, or my seat reclined an additional degree, he'd be right - it took a bit of playing to get them set-up. One effect is the panniers noticeably bend outwards when looked at from the rear. This does not appear to affect their attachment, but I'm considering mounting a bar inside to relieve any stress.
Other issues - the top mesh pockets have zippers & no pleat, thus lying almost completely flat & having little room for anything. Putting a set of keys in them is a struggle, and a 2-pack of Fig Newtons gets totally crushed. A velcro closure & pleat would make them much more functional. I've ridden off a few times with one of the straps unbuckled, which then drags on the ground. A simple piece of material 1/3 of the way up holding the strap to the bag would prevent this. Given the size of the interior, an interior (divided) pocket would be helpful. There's a strap that needs to be threaded thru the front of the 2 panniers; Ortlieb shows this strap going across the front of a narrow Zox seat. If your seat is a typical mesh one, this won't work unless you make a new, longer strap. Otherwise you thread them together behind the seat, which on the ICE is a pain given the low seat height & rack attachment. One advantage is that if someone tries to make off with them they'll never get them off. Reflective material placement is largely ineffective with a full load. There are additional small buckle closures at the front & rear of the top flap; when mounted the front one is almost impossible to get to. Finally, the material is wearing where it contacts the rack & there've been a number of times when I've removed the panniers and the rack inserts have remained on the rack (& yes, they've been put in correctly). I don't have as good a feeling about the longevity of these panniers as I did with the Arkels.
My perfect 'bent pannier - the basic Ortlieb design (long single compartment) made by Arkel (ie. their materials, construction, quality, etc). And there's always Arkel as a company - I'll put them up against anybody in terms of product quality, and their customer service is as good as it gets. Why do I feel like I'm talking myself into getting the RT-60's after all???
