It is not uncommon for length gains of an inch or two and even more as claimed by many dedicated PE practitioners. This typically requires an intensive weight hanging routine for more dynamic stretching of the anatomy. This too followed up with a supporting routine of exercises that include passive stretching 24/7 to help cement gains. A large part of successful penis enlargement involves self perception which is a very powerful thing not be underestimated.
There is a big difference between the mechanics of water vs air as a medium of inducing negative pressure or vacuum. With air having the capacity for expansion and contraction to acts as a buffer for safety or inefficient evacuation (?) with fluid transferring the forces of pressure directly for more efficient but less safe if not careful practice. Preference for one over another is subjective and therefore depends on the individual with both camps claiming superior results. I personally prefer a pump in which both air and water are available in modulation to explore the optimum for a given session. The biggest advantage in my opinion is that water temperature can used to help “warm up” the tissues for safer and better results. Again, one should have the ability for quick and efficient exchange of both mediums in a single device.