In selecting a vaporizer, you need to decide on whether or not you need something that blows cold air or warm air. Often times, especially if you speak with your doctor, he/she can advise which kind would be best for the type of cold or symptoms you are fighting.
I would also look at how easily the machine can be taken apart and cleaned. Water in many places will leave sediment or some kind of deposit. You will want to know if the manufacturer recommends something other than tap water, as some machines don't do well with these sediments. These deposits can also leave a ring in the vaporizer; the ring from the water would not concern me as much as bacteria that might be living in the water. So if you need to clean it each day, it's best to get something that is "user friendly."
Lastly, I would try to get something that does not require filters that need to be purchased. The vaporizer may come with one, but replacements may be hard to find. Filters will also collect sediment, and when wet, create an environment for things to grow.
I would try to do some research among brands to see which one is best based on these suggestions. I have had all of these experiences, and I know that once you buy a vaporizer, you'd like it to be the last one for a while.