I actually only got a Fhoenix because there was a problem with the SBS that they repaired once, and then replaced with a Fhoenix (customer service was amazing.)
The Fhoenix does have more of a twist feeling, feels a bit narrower on the widest horse, and is deeper and not quite as thick as the SBS. SOME of the SBSs the flap on the dressage almost angles back a bit, so for the wrong thigh type, your knee will go over very easily. The GP in both suited me far better. The Fhoenix runs slightly smaller, but I *think* they started doing them in more specific sizes, rather than just "Adult" I could be wrong on that.
His Princeness & the Pea never noticed a difference, and loffed both equally.
Have a few w-i-d-e body friesians. Did the whole treeless routine. The trekker and ansurs sucked because my legs stuck straight out and I couldn't get a leg on. Tried the HM SBS and it felt like I was riding on a sofa cushion. The one Duett I tried fit my horses like it was custom but I couldn't stand sitting in it.
Enter Laser 747. Ahhhh. Perfect. Made for wide bodies. Adjustable with a key. They have a website. Not terribly expensive -- considering. Seat for human also perfect. Has gel "flocking" and have never had a sore back. I'm done looking. Works for 5 of my horse of varying (wide) widths.