I have 4 Coway and 2 Winix purifiers and I can confirm: they're great sleep noise generators! The plastic front pieces and the filters themselves block/baffle any high frequency noise from the fans, leaving you with a pleasant low/mid frequency whooooooosh. I like it better than the sound from small cheap solid state machines; their small speakers can't do low frequencies.

    For most circumstances where someone would use 
    a white noise machine, this kind of small purifier 
    is all-around better. 

Depends if you enjoy fresh air when you sleep or not. If you've ever monitored CO2 levels you know how quickly they climb in a closed room.

But, if you are blowing fresh air into a room, or have the windows open, the filtration effectiveness of these units approaches zero pretty rapidly. And these air purifiers aren't exactly free to run: they use ~50-100W of power and if you run them 24/7 you can expect to spend $100+ per year on filters.

So there's no perfect answer...

> ~50-100W of power

That sounds too high? An AirFanta 3Pro on "2" is 141 CFM at under 5W https://www.jefftk.com/p/running-an-air-purifier-on-batterie...

That’s the design appeal of ionizers like BlueAir (not most ionizers, but specially those designed to supposedly prevent ozone production). They comparatively sip power while getting the same measured reduction in particulates and ozone.