This is a newer type of nebulizer. It is typically portable, though you can get larger non-portable models. When selecting the portability of your ultrasonic nebulizer, you will need to keep in mind your lifestyle, and whether you need a unit that can be on the go with you.
An ultrasonic nebulizer is designed to make a more uniform & smaller medication particle. Smaller particles of medication should be better able to penetrate lung tissue. Smaller particles should also be easier to breathe in. For those will allergies and asthma, such technology should provide an advantage, although the studies do not clearly indicate that at this time.
As a result, you can be paying out of pocket for your unit. In practice, most insurance companies and Medicare will not pay for these units because there has been no proven medical benefit of an ultrasonic nebulizer over a standard nebulizer in most user populations. The only user group where there has been a proven medical benefit of ultrasonic nebulizers over non-ultrasonic types is for cystic fibrosis patients.
So, do you need one? If your asthma or allergies are particularly severe, you could consider investing in the ultrasonic machine. However, check your insurance to confirm that you would get at least partial coverage of the more expensive machine. Otherwise, you could be out of pocket for a large amount of money.
Also, speak with your doctor. Your doctor may be aware of studies or ongoing clinical trials on these types of nebulizers. Preliminary findings are sometimes released within the medical community, when there is a benefit to patients. Therefore, your doctor's information could help with your decision. You might also want to take a look at allergy-related sites or online groups, which would allow you to find out what others experience with these units. Since the hard data is still inconclusive, it will be a decision that you'll make based on opinion only.