I would disagree (but everyone can have their opinion) but I would say that the S gets the better bumps over the non-S.
The 4S got Siri, double the RAM, and a fixed antenna design.
The 5S got the two-tone flash, and touchID and fixed the flaking aluminum.
The 6S got 2nd-gen touchID, double the RAM and a stronger chassis.
I also feel that the double RAM that the S sometimes gets would give better iOS performance on upgrades. I would expect that the 6S would handle iOS12 better than the 6 handles iOS11, but then this is just speculation.
My point is that when a new design happens, I'd rather wait for the next model. The X is a radical new design with new components. The faceID components will only get better on the next X. I guess the wireless charging might get better too, but that isn't a feature that I crave. I suppose, if it's not already too late, that the next generation batteries might deliver peak current better than the current batteries in order to prevent random shutdowns/throttling. In fact, the X might already have been designed for this as it has two separate batteries ... after all, Apple has known about this battery issue for close to a year and probably longer.