Well, I gained lots of experience in humidifiers
There are several things you should consider:
1- The capacity of the unit. It should be suitable with the size of the room.
2- Operating noise of the unit. If you will use it in the bedroom a noisy unit would be disturbing. (Ultrasonics are usually more silent than hot-steam types)
3- Water tank capacity. In my opinion the bigger is the better. Cos, frequent refills are not for the lazy

4- Refill should be easy. In most of the high-tec humidifier versions you remove the water tank to refill. The cap is at the bottom of the tank and not easy to open and very narrow. I hate them! Refilling without moving the unit or removing any parts of the unit is the best (i.e. by bottle or a jug).
5- When you use ultrasonic humidifier -apart from the fine dust that Karkunkoski mentioned- the minerals in the tap water became airborne and negatively effect the air quality in the room. If you have a sensitive nose like mine, you may end up using distilled water, which means more costs.
6- Hot-steam type humidifiers may be dangerous if you have little children around.
7- Lime quickly covers the hot parts of the hot-steam humidifiers, therefore requires frequent cleaning.
I used several humidifiers so far... Medisana Ultrasonic humidifier: Not efficient even for my bedroom and costly. Boneco (I don't remember the model code) hot-steam humidifier: I was very satisfied with that until it stopped operating after two months and I learned that it is a discontinued model and out of stocks... Bionaire hot-steam humidifier: Brought from the UK. Still using. Not very bad but refilling and cleaning is a pain in the a$$...
So, as a result, I would avoid any ultrasonic humidifier and buy an UFOX
