Deciding between 350cc and 375cc Moderate Round Implant. I'm looking for a natural "full C cup" What can I expect? (Photo)

Size of implants, the ones you have chosen are not very different.  In fact it is about 1.5 tablespoons.  With that said, your breast shape lends itself wonderfully with a textured round base shaped implant from Sientra.  These implants have a better appearance in a women with minimal breast tissue if your desire is a natural result.  The Mentor implants chosen will give you likely a C cup and be somewhat natural in my opinion.  For more certainty, consult and ask these questions again to your plastic surgeon. 

Thank you for the question and pictures. Base on your photographs, you are starting at a great place and should have a very nice outcome with breast augmentation surgery. In regards to specific size/profile of breast implant, despite good intentions, online consultants will not be able to provide you with  meaningful specific advice.  Ultimately, in person consultation with well experienced board-certified plastic surgeons will be necessary. Careful measurements, dimensional planning, and careful communication will be key.
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're looking for. Ask to see lots of examples of his/her work.  
2.  Again, have a full discussion and communication regarding your desired goals with your plastic surgeon. This communication will be critical in determining which operation and/or 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 who makes the bra; therefore, discussing desired cup  size may also be inaccurate.  The use of computer imaging may be very helpful during this communication phase.
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.  The use of temporary intraoperative sizers with the patient in the upright position makes selection of the best breast implant size/profile relatively easy.
I hope this, and the attached link, helps.

Choosing the correct size for your breast augmentation can
feel very intimidating. I can tell you that the difference between 350 and 375
cc, when both implants are moderate profile, is very minimal. It’s certainly
possible to achieve a very natural look without anatomical implants. My
recommendation is to visit a board certified plastic surgeon who utilizes 3D
imaging in his or her practice so you can compare how different sizes will look
on your frame. I also suggest bringing in some “ideal look” photos so your
surgeon has a visual reference to use as a starting point, as cup size isn’t a
standardized measurement.