There are lots of ultrasonics out there and the technology is basically the same for all of them. So, the kinds of things that might make my decision on a model would be
a) does the spout allow me to attach a standard size hose or pipe?
b) how easy is it to clean out the reservoir and unit? Weird shapes and sharp corners will make it that much harder.
c) can I use any kind of water in it (tap, distilled, filtered, hard or soft)?
d) the size of the reservoir.
e) what controls options are there (regulate it by humidity level, timers, pre-set cycling, etc)?
f) the units with ultraviolet light that kills bacteria might be a really good option as long as they don't heat the fog much. I've been watching to see if the prices come down on these. They probably will.
If the reservoir is too small I'll be taking the thing apart all the time to fill it. The more hassle it is the less likely I will be to get it done. If the reservoir is too big water will sit in it for a longer time raising the chance of bacterial or mold growth. The best water to use in the unit is distilled or reverse osmosis filtered, as there is less organic or mineral stuff being run through the system. Hard water will mean you need to clean the unit with vinegar as well as sterilizing regularly which is not hard. Soft water may add salts to the fog. If your water is chlorinated you will also have to remove residue that might build up to a toxic level or use dechlorinaters for aquariums.
I have used ultrasonic (cool mist) types for years and have not had problems with excessive mold growth or related bacterial health problems in my chams. I wouldn't think of keeping chams without a good humidifier handy now. I do still spray once a day for a good long drinking session, as the fog won't condense heavily enough for the foliage to drip for drinking. I don't know if the dripper would be enough...that might depend on how good your cham is at using it. But, these units have kept my cages humidified during the days when I can't be there to spray. If you plan to travel much and live in a dry climate or use central air or forced air heat I would get a misting system so you don't need someone to come to the house every day and spray. If you don't travel much and only have one cage I doubt it would be worth it.
