I can't speak to the Wunderlich or BMW bag, but can enthusiastically recommend the SW Motech Bags Connection City Bag which I have on my 2015 RT. Here's a link:
http://www.revzilla.com/motorcycle/sw-motech-quick-lock-evo-city-non-electric-tank-bag
It attaches to the tank via a tank ring, which keeps it off the paint, no straps to mess with and makes it easy to detach for gas fill-ups.
I use the Givi on my 1200 MUltistrada and they also use the ring mount. I think the ring mount is the best system for tank bags except for the older 1200RT mounts that clipped onto the bike. I also added a small piece of sheepskin at the rear base of the tank bag just in case.
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