High profile 300cc vs. moderate plus 285cc: can the high profile still give me that ski-slope look?​ ​

Hello and thank you for your question. You are a great candidate
for a breast augmentation.  The size, profile, and shape of the
implant is based on your desired breast size/shape, your chest wall
measurements, and soft tissue quality.  This decision should
be based on a detailed discussion with equal input from both you
and your surgeon.  This entire surgery
can be performed with a small incision technique.  Make sure you
specifically look at before and after pictures of real patients who have
had this surgery performed by your surgeon and evaluate their results.   The most important aspect is to find a
surgeon you are comfortable with. I recommend that you seek consultation with a
qualified board-certified plastic surgeon who can evaluate you in person.
Best wishes and good luck.
Richard G. Reish, M.D. FACS
Harvard-trained plastic surgeon

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 high and Mod (+), in reality, is very small (on the order of millimeters).  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 + 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, I try to get patients to commit to a particular profile first. This will determine whether they are seeking a more dramatic (HP) or understated (Mod +) result. Once we have narrowed down to a particular profile, we can look at options within that profile that are compatible with a particular patient's breast width/anatomy. The volume differential you are considering is slight (3 teaspoons) and is likely to contribute less to to your final result than the chosen profile.
As always, discuss your concerns with a board certified plastic surgeon (ABPS)