Difference between high profile and ultra high profile breast implants?

Thank you for the question.   Generally speaking, the difference between the different profiles of breast implants available have to do with the dimensions (diameter and height) of the implants. For example, the ultra high profile implant has a greater projection and a smaller diameter compared to the high profile implant. The moderate plus profile implant as projection and diameter characteristics that fall between the moderate profile and high profile implants.
 
 In our practice, I prefer to have a wide selection of implants (all profiles) available in the operating room.  "Dimensional planning" refers to the process of measuring patients' anatomic landmarks. I use intraoperative sizers and place the patient in the upright position to evaluate breast size. Use of these sizers also allow me to select the breast implant profile (low, moderate, moderate plus, high-profile, ultrahigh, extra...) that would most likely achieve the patient's goals. The patient's goal pictures are hanging on the wall, and allow for direct comparison. 
I have found that this system is very helpful in improving the chances of achieving the patient's goals as consistently as possible.
Ultimately, careful selection of plastic surgeon and careful communication of your goals (in my practice I prefer the use of goal pictures, direct examination/communication in front of a full-length mirror, in bra sizers, and computer imaging) as well as careful measurements (dimensional planning) will be critical.
Generally speaking, the best online advice I can give to ladies who are considering breast augmentation surgery ( regarding breast implant size/profile selection) is:
1. Concentrate on choosing your plastic surgeon carefully. Concentrate on appropriate training, certification, and the ability of the plastic surgeon to achieve the results you are looking for. Ask to see lots of examples of his/her work.   Sometimes, it is necessary for patients to seek several consultations before they feel comfortable with their selection of plastic surgeon.
2. Have a full discussion and communication regarding your desired goals with your plastic surgeon. This communication will be critical in determining breast implant size/type/profile will most likely help achieve your goals. 
In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. For example, I have found that the use of words such as “natural versus bolt on look” or "C or D cup” etc may mean different things to different people and therefore prove unhelpful.
Also, as you know, cup size varies depending on him who makes the bra; therefore, discussing desired cup size may also be inaccurate. Again, the use of computer imaging has been very helpful during the communication process, in our practice.
3. Once you feel you have communicated your goals clearly, allow your plastic surgeon to use his/her years of experience/judgment to choose the breast implant size/profile that will best meet your goals. Again, in my practice, this decision is usually made during surgery, after the use of temporary intraoperative sizers.
I hope this helps. Best wishes. 

Profile terminology will vary from one manufacturer to the next. The terminology matters less than the correlate dimensional measures.
As one goes up in profile the footprint or BWD of the device will shrink and the projection will increase for a given volume. I have found that full profile Allergan implants correlate closely with high profile Mentor implants. These devices exhibit higher projection than similarly sized moderate (+)/moderate profile devices. Ultra high profile is simply the next step upwards in projection.
Patients are encouraged to stay within the boundaries defined by their BWD. This is the best way to avoid the dreaded "fake" look. I generally recommend that patients with smaller frames who are seeking more than a modest augmentation consider a higher profile device.The higher profile device will impart greater projection and thus a less subtle result but will allow for a more significant augmentation than a similarly sized Mod/Mod+. Thus, the ultimate question is one of volume. Is the volume you can achieve via a similarly sized Mod + profile implant adequate to reach your desired goals? If not, a HP will afford you more volume at the cost of subtlety.
The difference between higher and lower profiles for a given volume is projection and for a given width is volume (with the high profile implant being greater in both scenarios). The moderate plus is the most commonly used profile in my practice. It allows for the most reasonable volume in given BWD. In my practice, low profile implants are the least commonly used (widest and flattest). I will typically reserve high profile devices for smaller framed individuals or those seeking a less subtle more dramatic change (the opposite of natural). 
Smaller framed patients tend to prefer HP implants as their BWD can be restrictive with regards to volume. An HP implant will provide greater volume in the same BWD footprint. But remember "natural" while difficult to define often implies a good match between frame, volume and projection. If the goal is natural more conservative sizing/volumes are recommended.
In general, the difference between a moderate and Mod (+), in reality, is very small (on the order of millimeters). Anecdotally, most feel that moderate profile devices are too flat. In my practice, I generally classify profile based upon patient preference. A higher profile provides a more dramatic look (size for size) vs. a moderate/moderate + profile which is relatively more subtle and understated. That being said the vast majority of patients will select a moderate + profile device.
With regards to your specific question, ultra-high vs. high will deliver greater projection and volume for a given BWD. There is a trade off of size for subtlety, as the ultra high profile device will exhibit a relatively more dramatic result as compared to the high or moderate plus profile devices.In my opinion, the key to a "natural" look is conservative sizing with an eye towards anatomy and frame. Smaller framed individuals by "nature" do not typically have very large breasts. The value of a higher profile device is allowing a smaller framed patient to achieve a greater volume without overstepping the bounds of the BWD.
As always, discuss your concerns with a board certified plastic surgeon (ABPS).