The difference between the 3 models is voltage and convenience. This applies to the Rife machines as well. If you are dealing with a serious issue like cancer, Lyme disease, Hepatitis C or even some arthritic conditions, etc. I would suggest using one of the 16v machines. That would be either the Zapper Ultra or the Zapper Professional The additional voltage helps to get the signal into the internal organs more efficiently, and if a terminal illness is involved time is a factor. The difference between the Ultra and the Professional is mostly convenience. The Professional will allow you to program in 3 different issues, so in Group1 you may have it set for Arthritis, Group 2 maybe colds and flu and then something else in group 3. Then all you need to do is select the group you want to use. The Ultra can only hold 1 frequency set. Either machine is simple to program – much like a digital clock and only takes a minute or two. The Professional also has a few additional functions, the most notable would be the Pulse Sweep. The Ultra has a SWEEP function that will run through all 10 of the frequencies in that set 10min. at a time. So, 1 full treatment is going to take 100 minutes. You can pause it and come back to where you left off and you can sleep with the machine and it will continue to SWEEP until you stop it. The Professional has the same SWEEP function as the Ultra but also has The PULSE SWEEP function. This will run through each of the 10 frequencies 1 second at a time – until you stop it. Below is some additional information on this function: As explained in the Book, the “Pulsed Sweep” was perfected by Dr Robert Beck, who had a massive following, and designed some very innovative Electro Therapy Device (machines) : Page 18 of the Instruction Book: The Pulse / Sweep function is identical to the “SWEEP” mode, with the exception that the machine Pulses the Group’s 10 frequencies every Second, rather than the normal “Sweep” Play-back which is 10 Minutes per frequency (non pulsed). Pulse uses a unique processor to “Store” and “Build” the Energy in a momentary Off Position, only to Pulse this Stored Energy back into the body in a quick Sweeping motion; increasing the effectiveness of the Frequency and focusing the Waveform and moving the energy deeper in the body for more efficient healing treatments. Dr Beck discovered this method by converting one of his original “Rife” Devices & moving circuit output through a Canon Camera Pulsed Flash, removed from an old camera. Dr Beck found that treatments were far more effective using the Stored energy Pulsed output, than via the standard fixed output methods. The same Pulsing methods are used in your Pro-Device, using modern microchip technology. All 3 models are effective and we have had great results with all models. Depends mostly on your budget. The Professional machine is really nice, if you can afford it I would do that. HealthProducts2