I'm a 34B. Which implant size should I use to achieve a full C cup bra size? Natrelle style 15 size 371cc or or 397cc? (photos)

Cup size is not determinable pre-operatively. Bra companies do not consult with Plastic Surgeon when they are manufacturing their bras. I find it more helpful to size the patient during the consultation to determine what degree of fullness she would best be pleased with . This takes a little bit more time and participation by the Doctor and his staff. But in the long run produces more happy patients since they are actively involved in the implant selection process. Because of your height and weight I would focus on HP Natrelle Style 20

Thank you for the question and pictures. I think that you are starting at a good place and should have a very nice outcome with breast augmentation surgery. There is no accurate/reliable/predictable correlation between size/profile breast implant utilized and cup size achieved. Ultimately, 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, and computer imaging) 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.
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 "full C cup” etc means 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 (and the attached link, dedicated to breast augmentation surgery concerns) helps. Best wishes.

The best way
to determine the size that fits your personal perception of the perfect breast
is to try on sizers. This way you can see how they fit on you, how they add to
your current volume, and whether they fit your frame. Although the surgeon can
guide you, only you will be able to tell what is the right size. Try on sizers.
Measuring a diameter and then telling the patient what volume they can
have (in high, moderate or moderate plus implants) in my mind is backwards. The
patients should be allowed to choose the volume. Then the surgeon carefully
considers the base diameter, projection, profile, manufacturer, and fill
material that will achieve the patients goals of size as well as qualitative
look (natural, intermediate, or bold upper pole fullness or projection), all
the while taking into consideration the patient's anatomic features that will
affect the outcome. The experience of the surgeon and the degree to which
they explain the options to you is of utmost importance. The subtlety in the
final outcome is not achieved by which profile implant is used, it is
determined by the skill in the creation of the pocket, the choice of implant.
Don't let the doctor tell you what would look good on you. They cannot tell
what you perceive to be the best size.  That is up to
you. Either 371 or 397 cc may make you a D cup. You need to be sure that these sizers look the way you want them to. I might be shooting more in the 300 to 330 range for you based on your concerns.