Either implant will look nice on you. Discuss your goals in general terms with your plastic surgeon, and he or she can help you decide. I do think that, with downsizing, you will probably need a mastopexy (breast lift). This will require more scars, but will provide an improved shape, and will also allow correction of your minor asymmetry. If your implants are above the muscle, then you might wish to consider placing the new implants below the muscle. This will disguise the implants better, and would also probably give you a less abrupt transition from your chest to the upper aspects of your breasts.
Sometimes, if the implants are to be changed to submuscular positions, then I advise the patient to consider a two stage approach: Take out the existing implants, allow some healing time to let the breast tissues readhere to the muscles and then later place new submuscular implants. Understandably, most women don't want to be without their implants, but this does reduce the need for possible breast lift. If your implants are already below the muscle, then the breast lift will improve the upper pole contour. Discuss all of the above with your plastic surgeon.