For me the advantage of the Wilderness over the onyx is 2 fold. I have ordered my Wilderness and will be selling my 21 Onyx.
1st is the roof rack, the stock rack on the onyx is not very user friendly in my opinion.Yes you can buy a Thule or Yakima rail system to replace the stock cross bars and that works fine for most application, in my case I am looking at putting a Hard Shell tent on top. It's been done but for me its causes the tent to sit to high about 4"+, and the weak link in those racks is the way they attach to the stock rails which are plastic. The dynamic load rating for the Onyx is 165lbs , cross bars are rated at 150 lbs, (could not fine the static load rating but I read 220 lbs). The Wilderness dynamic load is 220 lbs and static load is 700 lbs. I am building custom roof cross rails that are only 1" high from extruded "T" channel Aluminum that way my tent will be supported by the roof rack, it's going to cost me less than $200 for all of the pieces and hardware. I want the tent to be solid and with very little flexing due to bouncing down crappy roads.
2nd the X-Mode, the Wilderness's X-Mode will not disengage when you exceed 25 MPH in the snow and Mud mode. Before if you were spinning your tires and the wheel speed exceeded 25MPH the X-mode would disengage. That's not good if you are in mud or snow.
My intent is to use the Outback for short off-road trips 1-3 days where I would usually take my Colorado ZR2, because I can not store my truck with tent attached in my garage due to the door height. Where as the Outback with tent will clear by and inch, even after I change out the springs and add 245/65 R17 Falkens Wildpeak AT3W tires. For longer extended trips the ZR2 is by far the better options due to the crazy places I tend to wander to.
In my case the Wilderness makes sense, but each person has different needs and intent, just mall crawling would not make any sense in buying one.