Would 400cc Mentor high profile achieve this full but natural look? Would Allergan Natrelle or Mentor be better? (photos)

Thank you for the question.  Based on your photographs, I think that you are starting at a good place and should have a very nice outcome with breast augmentation surgery (assuming a well executed procedure).All of the FDA approved breast implant manufacturers provide good products.   Personally, I think that careful selection of plastic surgeon and careful communication goals are  the two most important "factors" when it comes to achieving the desired outcome with breast augmentation surgery.   The specific breast implant manufacturer and even type of breast implant are far less important factors, in my opinion.   
 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 to seek several consultations before you feel comfortable about your choice.
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. I have found that the use of words such as “natural” 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, best not to discuss your goals and/or judge the outcome of the procedure performed based on achieving a specific cup size. The use of computer imaging may be very helpful during the communication process. The use of in bra sizers may also be helpful. In other words, use as many “visual aids” as possible during the communication process. I encourage patients to meet with me as my times as necessary, to feel comfortable that we are both on the “same page”.
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. I generally select appropriate breast implant size/profile after the use of temporary intraoperative sizers and viewing the patient's chest in the upright and supine positions.
I hope this helps. Best wishes for an outcome that you will be very pleased with.
 

Both Mentor and Allergen can provide the results you are looking for, ultimately choosing the right size will depend on your actual measurements. To answer your question about size and how many cc’s you should get based on the size you want to achieve, first you need to take into account what's known as your base width diameter.
Imagine lying on your back and drawing a circle around your breast. If you were to draw a line across that circle, it will be a given length. Your plastic surgeon will measure this length, which is called ‘’base width diameter’’ or BWD for short. For women, the base width diameter can go from 9.5 cm to 16.5 cm, or 3.75 to 6.5 inches. As you can see, there is a very wide range. 
For example, a 300 cc implant on a woman with a BWD of 10 cm is very different than 300 cc for a woman with a BWD of 15 cm. On one it may be enough volume, and for the other it may be much less than the desired result. 
As far as the placement of the implants, placing them under your muscles - this is still the most popular technique for a variety of reasons, one of the biggest advantages is that it prevents rippling and it also hides the implant better. It is probably still recommended to replace the implants under the muscle, but a discussion of your goals during the consultation will help determine the final position.
I hope that answers your question on size. It’s great that you’re doing this research. I always say that the best patients are informed patients.
Thank you for reading and best of luck on your journey!