I've recently become very enamoured of the KISS RB and although finding the money for the unit is the big issue for me, I'm "selling it" to myself on several "safety" aspects which as a "mature" diver may probably be relevant for you to Simon:
1) constant high ppO2 reduces physiological stress
2) depending on the unit, very lightweight (the KISS Sport RB anyways)
in comparison to a twinset, therefore reduced physical and physiological stress again
3) ease of use, you could use the rug for a whole day's diving without worrying about fills and changing the scrubber
4) will reduce gas costs over a long period, especially if you're wanting to dive high % nitrox regularly (ok so Sofnalime ain't free either but...)
With so many divers bowing out on a heart attack, I think reducing physical stress levels can only be a beneficial move for anyone in the "mature" bracket - at least if I cark it while working down at the gym I won't go undiscovered or become crab food
Steve (oops! on Jules's log in)