Our experience is with Toy Haulers, but we have a Keystone Impact by Fuzion, which I would say is their "mid-grade" model. Right after we got ours, our neighbor went and bought an actual Fuzion, their "high-end" model, in about the same size as ours. The only differences I've seen so far is that they have way fancier stuff, like bigger TV, extra speakers, fireplace, nicer countertops, bigger fridge, fancier light fixtures, fancier cabinet hardware, fancier bedding, and massagers in the couch (ours are just heated). Oh and their exterior steps are nicer (which we are already planning on replacing). Also their systems are controlled via a touchpad that he can operate from his phone if needed. They also have an extra half bath, but we don't have kids and they do so that's neither here nor there IMO. Our main bathroom setups are exactly the same, only their counters are solid surface and ours are laminate. Even the door is the same.
But when you look at stuff like the "guts", ie, dump valve setup, leveling system, generator, etc., they're all exactly the same as ours. All I could think when I was looking around his was "all this stuff's going to break at some point, and it'll just cost more to fix it". I also know that our tie down systems in the garage are the same. All his extras also make his trailer much heavier, and also mean his carrying capacity is less.
That being said, if we were going to full time, I'd probably go for the fancier one. But this is just a weekend/vacation trailer for us, so what we have is exactly what we need/wanted.