The TM and Onyx are the two saddles I keep going back and forth with, but mostly gravitate to the TM. This is all because of the lower D's that the bridge runs through. My lanyard lives there full time (I absolutely can't stand the pain I get from hip D's), and the TM has the best interface for this.
The Onyx is a very close second, although I personally couldn't go without upgrading to Rock Exotica rigging plates. I run the plates with the large eye facing out front, and the three eyes (from bottom to top) hold leg loops, waist loops, and bridge.
The Onyx adjusters can take some time to get set to your liking, but once there, I've found they stay put by tucking the remaining strop back through the adjusters. This saddle will also host a second bridge in a much cleaner, less cluttered manner than the TM, mostly due to the multiple holes on the rigging plates.
My one real gripe with the Onyx is how it hangs away from the base of my back. The shape of the TM makes full contact all the way around my waist. This sounds like a small detail, but I did once get a small chunk of wood down the back of the Onyx and that hurt like crazy until I could get into position to safely manage it. It's never happened with the TM.
In all, I honestly think both saddles are fantastic. Very comfortable for long days in the canopy, and very adjustable. The Onyx comes in multiple sizes, so that may be a plus. If the saddle is sized properly, and it still doesn't work for you, you might be missing on a crucial small adjustment that will change your experience for the better.
As far as the saddles twisting out of place, the TM is more resistant to it, but the Onyx always seems to fall back in place once you can unweight it for a split sec. Keeping the saddle as low on your body as you can, with a longer bridge will allow more latitude for side motion on the bridge before it really wants to pull the saddle out of place.
For some people, finding the right saddle can feel like a life long endeavor, but one that pays off huge when it becomes a reality.
Best of luck!!!