I am new to saddle hunting, so take my opinion with a large grain of salt. Like, rock salt...
I had also been looking at the JX3 Hybrid, but ultimately ordered the Mantis for a couple of reasons.
First, it seemed to me that if you are sitting in the JX3 and a shot opportunity comes, you will have to shoot from the sitting position. You can't just stand up like in the competitor saddles. This was a deal breaker for me by itself.
Second, I have a neighbor that owns the Mantis, and have had a good long chance to test it out and look it over thoroughly to know exactly what I am buying. I am still waiting for my Mantis and Predator to ship, but I am not worrying for three months that I might not like it. If I had ordered the JX3, I would be biting my nails every day for a few months not knowing if being locked down in a sitting position would be something I could live with or not.
If I received my JX3 in July or August and not liked it, I would have to return it and then order another backordered saddle like the Mantis, which would then arrive in October or November. On top of that, I can use whatever pack I want with the Mantis, whether it is a small day pack or a full sized ruck, depending on my hunting situation. Just my 2 cents...
I've never actually sat in the JX3 though, so maybe I am completely wrong.