How much noticeable volume between 350cc and 385cc? Sientra HP round.

The volume of these two implants is quite close, 35 mL. That is slightly more than 2 tablespoons of water. The differences between the two is small and making a decision can be difficult. By your height and weight, most likely both of these implants can be nicely placed by an experienced surgeon. I would not expect a lot of side fullness in most patients. Although the appearance between these two sizes is quite small, often the decision-making can be anxiety provoking. You should be able to have a good discussion with her surgeon regarding your concerns, either being too large or too small. My best advice is once you have made your decision, do not second-guess yourself as often this leads to more anxiety.

In most cases, a 35 cc difference does not create a clinically significant difference.My best advice: more communication.
It is definitely worth spending more time (prior to the date of surgery) with your plastic surgeon to discuss further.  As you know, it will be important for many reasons, that you feel comfortable that you and your plastic surgeon are on the "same page" prior to proceeding.  Show your plastic surgeon what your goals using as many visual aids as possible.   As you can imagine, terms such as "natural" or "side boob" can be interpreted differently by different people.  One question that might be helpful for you to consider: would you be more disappointed if you are slightly too big or slightly too small after the breast augmentation procedure?
  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.