This may be a somewhat controversial statement, but in my opinion there currently is really only one acceptable position for breast implants in an elective cosmetic surgery patient: behind the pectoralis major muscle (sub-pectoral augmentation). There are a number of very compelling reasons to place implants behind the pec major, and the most compelling one of all is the fact that radiologists report that the mammographic imaging of breasts for the purpose of breast cancer screening tends to be more easily accomplished when breast implants are sub-pectoral (compared to pre-pectoral, also referred to as the 'sub-mammary' position). An American woman's current lifetime risk of breast cancer is approximately 1 in 8 to 1 in 9, so the issue of breast cancer screening must be taken very seriously. Mammography is by no means a perfect screening study, but it is the standard of care at this point in time. The most sensitive and specific test for breast cancer is a contrast-enhanced MRI scan, and breast implants do not impair breast tissue visualization by MRI.
The next very compelling reason to select sub-pectoral placement is a cosmetic one. Implants placed on top of the pec major tend to stand out in the upper pole of the breast, creating a rounded, convex and distinctly unnatural-appearing breast profile. With implants in a sub-pectoral position, the upper pole of the implant is flattened somewhat by the muscle, helping to create a smooth transition from the area in the upper chest where the breast begins, and a gradual slope towards the nipple that is not excessively rounded or convex. In some patients with a fuller breast volume preoperatively one may get an acceptable appearance with pre-pectoral placement - initially. The problem is that as breasts age, the fatty tissue atrophies and breast tissue thins out, and the area where this is most obvious is in the upper pole and cleavage area. So a pre-pectoral implant that was initially well-concealed may, after a few years, become painfully obvious (including visible implant folds and ripples) in the upper pole.
'Under the muscle' and 'sub-pectoral' are actually somewhat misleading terms, as in most cases the implant is only partially subpectoral. The anatomy of the pectoralis major muscle is such that it is actually just the upper/medial half of the implant that is covered by the muscle, while the lower/lateral half of the implant is submammary. The pectoralis major thus provides an additional layer of tissue to conceal the implant in the most cosmetically significant area of the breast: the cleavage area. This is why saline implants are often easy to feel laterally, as they are covered by breast tissue only in lateral aspect of the breast, and in slender patients who have small breasts preoperatively the implant is often immediately under the skin in this area.