Since I'm a newbie here, I have no idea "who rubs who's back" here, who is cheeky, and who knows what they are talking about.
But I know machines, and I tore apart five Kirbys in the scope of six weeks, refurbished them, and I'm using them DAILY (not all five on the same day obviously).
I have no idea who "Marcus" from Cardiff is, but I agree that Ultimate G Diamond IS the best (for me!). I also must add that I never used Sentria (I've seen one in the pawn shop, but I was not handling it at all, just looking). The wobble I have on my G4 and my Diamond, I attribute to "one connecting point" vs. two (on older Kirbys). The ones that have the pin through the handle, connect the handle fork with the motor at TWO points (one on each side). No wonder this connection will feel more positive, than one contact point in the middle of the mass of the motor unit.
This is the ONLY gripe I have with the Diamond, and I LOVE the speed switch. If someone can not tell when the machine is on reduced power, they are either deaf, blind, or high (or all of the above at the same time!).
Now,....if Sentria doesn't have the "wobbly handle syndrome" (I don't know), it would be a GREAT improvement, but the fact that it has too much plastic on (which I detest), and it looks like something Dart Wader would vacuum with, keeps me away from it.