One BIG, BIG advantage over the other seat is that the seat bears on the tree at shoulder level meaning your centre of gravity is kept low and I only use a tree strap with the extensions on, the X3MI seat looks inherently unstable with the tree bracket at bum height! Now sitting on it rather than in it would make me nervous of moving!! I've stood on the seat of mine to trim some branches above it, don't think I'd do that on the X3Mi one.
To take a shot at your left or right limit I put a foot down a rung allowing me to swivel and take what is effectively a standing shot off the rails. (A quick look at the other seat suggests this might not be so easy with the angled rail).
As for portability..... seat over one shoulder, rifle over the other, stick, foxpro etc etc. Have also carried the extensions and the legs on occasion.
I ordered it from Bushwear and waited 2 months for the Cub extension to arrive. They did keep me informed via email and didn't charge but it was a tad inconvenient at the time.