I would recommend getting a model that has a lever vs. a button. I have the button variety and believe it is not as efficient as the lever.
I've been counting my output and am consistently getting fewer bottles per cylinder than advertised, so I consulted an expert. She blamed the button. Less consistent pressure = more waste.
Oooh, interesting! Seriously. I was thinking that I was getting fewer bottles than advertised, but I also make my water super-mega-fizzy (more than 3 of those loud noises, which we have dubbed "farts" -- I usually go for 5 or 6; the problem with that, such as it is, is that all other commercial carbonated beverages [pop from a fountain, pop in a can] always seems flat to me).
I just assumed my low yield was because of my hyper-carbonation. And it might be. But now I'm intrigued about the efficacy of the button vs. lever. Hmmm.