when on top I use the tiedowns and run a strap through the back wheels to keep them from moving around. I also use velcro straps to affix the front wheels to the bikes to maximize storage space under the cover.
Just showing off some of the ways to use the T-slot rails on the Retrax XR cover. The biggest thing I wanted to do was to load bikes on top of the cover while keeping luggage dry underneath. I made a simple rack using 8020 extrusion and carriage bolts to fix in the T-slots. I also made tiedown...
I have Retrax XR. It is so fast and effortless to open that I cant imagine the benefit of using the powered version. One may argue that if your hands are full you can just hit the FOB but reaching for the fob is an equal effort to just popping the lever. not to mention, now you have to carry...
I have the Retrax pro XR and confirm the pros and cons above. another con though is that you can open the cover while the tailgate is closed and locked. this means you have to lock it with a key if you want the bed secure.