The onyx is a very different saddle. I have a Petzl Sequoia and a new tribe onyx that I just bought. The sequoia shared a lot with the TM, in design, but imo, the tree motion looks better quality and comfort than the sequoia.
As for the onyx. I think it has some really great features. I love the triangular leg pads, the wide back pad is nice, and it has more gear storage options than the TM, in stock fashion. I also feel the onyx would make a good production harness, it seems extremely durable and bombproof materials are used. With that said, I’m sending my onyx back to treestuff. The way the front of the bridge is designed, it made the leg pads ride up and dig in in the junk, and the back pad was too thin and bit into my hip. The onyx would be an incredible harness, but lacks padding. I really don’t think it has any padding whatsoever. I’m a thin guy and have been struggling to find a harness that I like. I will be trying a monkey beaver and then a tree motion if I can’t find comfort in the monkey beaver