I am debating between 250/275cc or 300/325cc high profile dual plane implants - which size would be best for me? (photos)

Choosing an implant size/style is an important decision which is best made in concert with your surgeon. Ultimately, options are based on a number of factors including:
-Chest dimensions: This point is crucial as the width of your breasts defines the available "real estate" on which the foundation of your augmentation is built. This "foot print" is the absolute limit with which you are dealing. Too "large" is not so much the risk as too "wide." You cannot build a 3 acre mansion on a half acre plot. An implant which is too wide (exceeding the available space on your chest) will generally over hang laterally making it more noticeable and occasionally contributing to discomfort and nerve symptoms (numbness or tingling). This will also contribute to a more unnatural look.
-Tissue quality: This relates to size in the sense that the implant will be suspended by your tissue. Too large an implant may place took large a stress on these suspending tissues leading to healing problems in the short term and bottoming out in the long term.
-Patient desire (what is the look you are hoping to achieve?)
My role as a surgeon is to manage expectations and guide you towards a prudent choice which can satisfy these reasonable goals. Often times, a surgeon can guide a patient toward a size/cc range and patients will make determinations on implant style/profile. Patient seeking more or less projection, or seeking a more pronounced or subtle upper pole volume increase can be guided toward an implant which will help them best achieve their goals. Additionally, technique variations (sub muscular vs. sub glandular) can also aid in obtaining the look you desire.
There are a number of tools available to help guide patient/surgeon decision making including sizers, 3D imaging and goal photos. Each modality can help better communicate your goals to your surgeon and vice versa.
Ultimately, it is important to not marry oneself to a particular "cc" as oppose to a particular "look." Communication is key as you and your surgeon create your custom surgical plan.
Cup size is difficult to predict. Your options do not seem unreasonable. A more "natural" look is often associated with lower volumes and profiles. The differences between these options will be subtle. Remember 45 ml approximates the volume of a shot glass.
As always, discuss your concerns with a board certified plastic surgeon.

Many surgeons have sample implants available you can "try on" to get a feel for how you may look after surgery. Either choice should give you a nice result. The 250cc implant is a bit more conservative. If you are wanting a noticeable change you may be happier with the 300cc implant.

The difference in volume between the implants that you have been offered is not all that important, it is really the dimensions that matter (i.e. the width and projection of the implant). With either of these choices, you will notice a different in breast volume, but different implants will give you subtle differences in shape. You could consider a consultation with the use of 3D imaging to help you make a decision alongside your surgeon.