Which implant size do you suggest would be better on me?

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.  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, in bra sizers, and computer imaging) as well as careful measurements (dimensional planning) 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. For example, I have found that the use of words such as “natural” or "sexy" 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 for an outcome that you will be very pleased with. 

Thank you for your question.
Before you make your final decision on size and shape of implants I would
suggest that you meet with your Board Certified Plastic Surgeon again and go
over your concerns and what results
you desire. Your surgeon should be able to explain to you why they are
recommending the size they are according to your current breast measurements
including your chest width and your desired outcome. It may be that a larger
implant even though the difference between a 250cc and a 315cc is very slight,
will fill up your existing skin and give you a more augmented look rather than
the natural slope you desire. Your Surgeon should be able to advise you
thoroughly on your options. Most important is
to pick a Board Certified Plastic Surgeon - Certified by the American Board of
Plastic Surgery. It can be hard and confusing for consumers when a physician
may tell you they are Board Certified. Make sure it is Board Certified
in Plastic Surgery as many Physicians are certified but it is in another
area such as Dermatology, Gynecology, ENT, or some other specialty that does
not have the years of training and Certification in Plastic Surgery required to
be a Board Certified Plastic Surgeon.
Rather than finding a non-certified surgeon that may give you better pricing it
can be helpful to find a newer Board Certified Plastic Surgeon that has years
of experience in training and surgery-required to become Board Certified in
Plastic Surgery but is newer in practice. They have the latest up to
date training and highest standards as currently set by the newest
technology and are able to pass on to the consumer much better pricing for Breast
Augmentation. The manufacturers of the breast implants encourage and assist the
newer Certified Plastic Surgeons by giving them special pricing on implants
which most will pass on to you, the consumer. Check for special pricing here on
RealSelf and make sure the surgeon is a Board Certified Plastic Surgeon and you
may find the perfect Plastic Surgeon for you who is able to work with your
budget without cutting corners!