I am debating if I should go with 350cc or 325cc MP+ Mentor Implants and mastopexy. 5'6, 27 years old and 120 lb. (photos)

The difference between the sizes represents less than 2 tablespoons, or one notch on your bra strap. Either size would work. You should have further conversations with your surgeon regarding your concerns with mastopexy. I recommend an in person consultation with a plastic surgeon to discuss your goals and be measured. After I do this with my patients, we then look at pictures of women with similar measurements with different size breast implants. My patients tell me that this process is very helpful in determining what size breast implant is right for them.
Dr. Ted Eisenberg, Board Certified Plastic Surgeon and Author

Hello and thank you for your question. You are a great candidate
for a breast augmentation.  The size, profile, and shape of the
implant is based on your desired breast size/shape, your chest wall
measurements, and soft tissue quality.  This decision should
be based on a detailed discussion with equal input from both you
and your surgeon.  Make sure you
specifically look at before and after pictures of real patients who have
had this surgery performed by your surgeon and evaluate their results.   The most important aspect is to find a
surgeon you are comfortable with. I recommend that you seek consultation with a
qualified board-certified plastic surgeon who can evaluate you in person.
Best wishes and good luck.
Richard G. Reish, M.D. FACS
Harvard-trained plastic surgeon

The 25 mL that you are struggling with represents a very small difference.   You would likely not be able to tell much of a difference between the two once implanted. Regarding the mastopexy, I would give serious consideration to your surgeon’s recommendations. If you are uncomfortable with the recommendations you can seek a second opinion.  Although no one really wants the additional incisions from a mastopexy, it is often the best option to achieve a nice breast shape and appearance.  Best wishes.