The Apex X-Plate in UPM or Allflex mount flavors function the same*. The rear is a hinge, the front is a sliding hinge. I don't think one is better than the other, they just want to provide the options.
*The X-Plate with Allflex mounts gives you the option of adding brackets in the middle of the plate that limit its vertical travel away from the board. That would result in a different ride.
As for @BlueB's and @lonbordin's comments, I agree in the legal sense but disagree in the moral sense. That's very theoretical and subjective of me, and is just my opinion. I can't quite define IP theft, but I know it when I see it, and it turns me right the hell off, personally. I would never buy the knock-off Geckos or Allflexes on principle. A Coiler or Donek on the other hand, is not exactly ripping off a Kessler, because Coiler and Donek can't know the entire recipe for the secret sauce. They are following the state of the art, and they are using their expertise and intellect to produce their original interpretation of it. That's honorable, IMO. Now, if Iron Rock advances the state of the art with a plate that functions similarly to an Allflex but is made of a much more durable composite, that would be original, IMO.