On one hand, hard luggage is, well, rather hard and rigid. This means the gear inside is likely to be well protected in the event of impact. However, those same impact forces need to go somewhere and they are more likely to be transmitted into the racks or motorbike frame itself where they can cause more costly damage. By comparison, soft luggage is more likely to absorb the impact and simply tear. Sacrificing the luggage is more preferable to me than sacrificing the bike.
On the other hand, hard luggage is probably more likely to survive a slide on pavement and thus may have an advantage over soft luggage if you're doing a lot of road riding. It also offers better security, which tends to be more of a concern when in paved areas than in remote trail areas.
Soft luggage suits my needs, so the question was what type? After reviewing several popular brands, models, and online opinions, the Wolfman Expedition Dry saddle bags at around C$275 (motorcycle show special) seemed to offer the best value for size, construction, and fitment to the WRR. The pair holds 38L which, in my experience (including more than a year of rough travel on foot and bicycle around the world) is plenty of storage for extended tours.
In any case there's plenty of room for another 35L+ of dry bag storage bungied to the top of the rack and it doesn't interfere with ride position.
The design of the Expedition bags requires a rack on the WRR to keep them out of the rear wheel and away from the exhaust. Moto-Racks in Vermont makes a decent custom rack and I included the option for a RotoPax mount. At around C$350 after taxes, duties, and shipping, it wasn't cheap. However it is well made from welded tubular steel, is surprisingly light (a few pounds), the owners were good to deal with, and I like to support small business even if it's not in Canada.
They sure didn't skimp on packaging!
Installation using the included hardware was straightforward. Nevertheless, I plan to replace all the bolts with button head metric versions in stainless steel to simplify my tool requirements.
This bar installed under the seat provides a mounting point for the right rack. Since the topmost bolt is shared with my homemade top rack, I had to use my existing M6 bolt rather than the supplied M8 bolt.
Fit is excellent, no forcing, drilling, or filing needed.
Installation of the RotoPax mount is also a piece of cake since the optional side panel I ordered are pre-drilled for the mount. This is a great location for the extra can because it lowers the weight distribution and balances the exhaust.
Unfortunately, I didn't consider that the side retaining straps for the left Expedition bag would no longer reach the rack! Turns out Wolfman sells an extension kit for exactly this reason, but none were available until June. Time to make straps!
Mountain Equipment Co-op sells webbing, 1" D-rings, and other harness goodies that are identical to the Wolfman materials. Materials for four straps cost me $2. I used 13" lengths of 3/4" webbing, but recommend using 1" webbing in 11-12" lengths for slightly better fitment.
It's a good idea to melt the ends with a lighter to prevent the webbing from unraveling. (Note that some cheap webbing is highly prone to unraveling even after the ends are melted. The good climbing-grade webbing doesn't have this problem.) Also be sure to flip over one side a half turn. This lets the strap thread through the rack and lay flat.
I sewed two bar tacks within an inch of the D-ring, tucking the main loop under the sewn flap. Use a heavy nylon or polyester upholstery thread and double back over your stitching to prevent the thread from unravelling. This construction is plenty strong.
Half an hour later, four straps for about 10% of the cost of ordering the Wolfman straps online:
They thread onto the left rack like this:
Now the bags mount up and cinch down perfectly over the RotoPax.
Update March 22, 2015: Out of desperation for spring (it's about -12C and windy) I've been ripping around the neighborhood and practicing 1st-gear skills in my driveway with the bags on. Positioning and stability seems good, and it's easy to move around the bike. Width isn't too bad either. The real test will be on the trail with bumps, etc.