250 or 275cc High Profile Round? Slim athletic build (Photo)

The size difference you are considering is less than 2 tablespoons. It won't be visibly noticeable when the implants are inside the body. In my practice I use sizers during surgery. This is especially helpful in a situation like yours. The sizers help me decide what fits best in the breast space and is most closely matched with the patient's surgical goals. I've attached a video that I hope you find helpful.

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.
Based upon your photos, your choices sound reasonable. 25 ml will be unlikely to make or break your result. That being said, anecdotally speaking, most patients wish they had upsized.
As always, discuss your concerns with a board certified plastic surgeon (ABPS).

Generally speaking, selecting slightly larger implants is always a good idea because the implants will settle slightly causing an apparent (but not actual) decrease in size. Furthermore, you seem to have some lower pole constriction, and a slightly larger implant will assist in expanding this lower pole. You are also correct, I would not stress over 25cc, you will look excellent either way!