What's your opinion on 500cc vs 550cc breast implants?

Thanks for your question. The difference between implants that you are interested is not large enough to make a significant changes in the final outcome. I always tell my patients that when they are not sure about two different implant sizes, always go with the larger ones. But I think this is a great topic for you to discuss with your surgeon and do not have an operation until you are happy with your implant choices. Good Luck.

Patients often think in terms of cup size when considering augmentation. Unfortunately, devices are sized in terms of milliliters (cc) of volume. This can lead to some confusion when sizing. Additionally, it is important to remember that cup size itself is not standardized with variations from one manufacturer to another. Unfortunately, as many women can attest their cup size in an industry leader such as VS is not necessarily transferable to another brand.
Another point which is often under appreciated is that of anatomy and starting point. Any implant will add volume to the volume which is already present. The implant is additive. A particular volume will not necessarily confer the same cup size to different patients (often times it will not even confer the same cup size to different breasts in the same individual...remember they are "sisters" not "twins"). 
A general rule of thumb is that 125cc can represent somewhere between 1/2 to a full cup size increase. Smaller volume differentials (25-50cc) are typically less consequential representing a volume change of less than a shot glass. However, I have found these numbers, at least anecdotally, to be of little help. Patients often present with notions/goals which do not correlate with these sorts of sterile volumetric assessments.
When sizing patients, there are a number of useful tools including:
-3D imaging (has the added benefit of offering a volumetric analysis of the pre-operative breast)
-Breast sizers (rice bags)
-Goal photos
I also recommend that patients commit to a particular look rather than a cup size. Once a patient settles on a look that pleases them the overall cup size increase becomes less relevant.The key to obtaining a natural result is to stay within the parameters defined by your BWD. This will ensure that you avoid the dreaded "fake" look. Regrets regarding size choice do occur. However, communication is key. Ensuring that your surgeon has a clear understanding of your goals/expectations is the best way to avoid these regrets.
As always, discuss your concerns with a board certified plastic surgeon (ABPS).

Dear Kaitlyn,   Front and side view pictures would have been helpful. You did not provide your present chest and breasts circumference measurements, that are critical for the correct implants size selection.   Based on your height and weight, let's assume that your are currently 36 small A cup size. 500 cc volume will bring you  to a 36 C + cup size. If you are 38 small A, you will become 38 C cup size. 50 cc additional volume, will make a small change only -  1/4 size increase.   If you are concerned about achieving your desired cup size, it will be safer for you to go with 575 cc - 625 cc according to your chest circumference (36 or 38).   Obviously, if you are currently an A cup size or bigger, that will change the whole outcome. So, I hope that your surgeon measured himself the chest and breasts circumference in inches.   Always, consult with experienced board certified plastic surgeons who operate in accredited surgery center for your safety. Most importantly, check the before and after pictures in the photo gallery, to make sure that they are numerous, consistent and attractive with nice cleavage, perky, symmetrical and natural looking.                         Best of luck,                                            Dr Widder