I am undecided between getting 350cc or 375cc Mentor smooth round silicon implants?

There is so little difference between those implants that you would likely not be able to tell the difference. What can make a big difference is the amount of projection that the implant provides as many implants now come in several styles all differing in projection.  You might want to discuss this with your surgeon as well.  Good luck.

Thank you for the question. 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 means different things to different people and therefore prove unhelpful.
 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.
 
3.  Once you have selected your plastic surgeon  carefully and 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, and the attached link (dedicated to breast  augmentation surgery concerns) helps. Best wishes. 

The best cosmetic result in any
particular breast augmentation patient depends on a variety of factors,
including: your individual anatomy, realistic expectations, a thorough
discussion with the plastic surgeon about the options, and an understanding of
the pros and cons of any particular implant choice. Proper sizing is not just
about the number of cc’s. The thickness of your tissue, breast dimensions which
include the width, height, and projection, as well as chest wall width all need
to be considered when choosing an implant. Trying on implant “sizers” of
various shapes and volumes while wearing a tight t-shirt, bra, or bathing suit
at a preoperative visit will help you and your surgeon choose the optimal
implant.There is minimal difference between a 350 and 375 cc implant.

Keep in mind that following the
advice from a surgeon on this or any other website who proposes to tell you
what to do without examining you, physically feeling the tissue, assessing your
desired outcome, taking a full medical history, and discussing the pros and cons
of each operative procedure would not be in your best interest. I would suggest
that your plastic surgeon be certified by the American Board of Plastic Surgery
and ideally a member of the American Society for Aesthetic Plastic Surgery
(ASAPS) that you trust and are comfortable with. You should discuss your
concerns with that surgeon in person.

Robert Singer, MD FACS

La Jolla, California