I have a "S1" and "S9" setup in my bedroom right now. Both are CPAP and *not* APAP. I'm a new CPAP user (two months)...
I find the S9 superior in almost every way. I see people admire the S1 for its hose swivel, power input swivel, ability to see water level from the front (?!?!)... like any of that *really* matters.
For me, the S9 is a diamond while the S1 is chipped glass.
The S9 is much more quite than the S1. This is my opinion and my spouses. Not only is the S9 quieter, but the noise that it produces is less annoying. The S1 produces motor (wining) noise while the S9 noise is simply the sound of air. Both machines are set to maximum exhale relief (I believe I already said I'm a new CPAP user).
I like humidity and my S1 always runs dry (actually, it always has about 30ml left in the tank; the heat plate is too small). The S9 never runs dry and provides excellent humidity (I'm using SlimLine tubing only and *not* the ClimateLine heated tubing). I also like the S9 humidifier preheat function.
I find the S1 pressure to "race" back to quickly. I'm half exhaled and it jumps to full pressure providing no exhale relief at all. The S9 exhale relief is far more like real breathing (for me) as it increases pressure with the inhale.
I could mention more things (I'll mention the YouTube video of a S1 sales rep. claiming/lying about their machine measuring humidity temperature at the mask because they know how much temperature is lost through the tubing... BS!!).
Obviously, I prefer the S9 in every way that *I* think matters. Unlike some others, I will say that I'm not affiliated or will profit from the sales of either machine.
I hope the above helps some folks that have searched so hard (as I did about two months ago).