When considering the purchase of a diffuser, consider why you want it. There are nebulizing, micro-mist diffusers and spa-type diffusers that have water and sometimes colored lights with music.
I feel the nebulizing diffusers (no water) are the most desirable for therapeutic, medicinal and germ killing effects. They will do the best job of filling the room with microscopic molecules that kill germs and are easily inhaled into the lungs. Videos on the detail pages show how easy the HEO models are to operate. Our family has the Whisper, but we don't move it around much so that works good for us. It's been very reliable and the controls allow us to conserve our use of the essential oils. They didn't make the PRO when we purchased the Whisper, or we probably would have gotten the Pro. I like that it's "all-in-one."
Click on the names of the diffusers to view the videos:

The Cool Mist diffusers that use water are typically used in spas, and some parents like them in a child's bedroom to diffuse nightly. These add a little moisture to the air. The essential oil molecules are carried into the air with the water; they float quickly to the floor as opposed to staying suspended in the air longer as with the nebulizing diffusers. I appreciate the suggestions for these models given, as I have very little experience with this type of diffuser.
I hope this helps! Blessings, Linda