First of all, you should take the advice of non-plastic surgeons (friends) with a truckload of salt, and the advice of plastic surgeons who have never examined you or spoken with you directly with at least a grain of salt. Your friends may have your best interests at heart (they may also be unable to filter jealousy or could be "frenemies"), but even the purest of intentions by your friends is without the training and experience of any ABPS-certified plastic surgeon. Also, individual experiences (anecdotal events) are neither scientifically relevant, nor indicative of "the right choice."
Plastic surgeons have the benefit of medical school, surgery residency, plastic surgical fellowship training, written examination, oral examination, American Board of Plastic Surgery certification, and years (variable depending on surgeon) of experience with numerous (extremely variable depending on type of practice, years of practice, and cosmetic, reconstructive, or mixed practice) breast augmentation patients. Some plastic surgeons are more scientific-minded than others, and even well-trained and seasoned plastic surgeons may rely on their personal experiences more than scientific studies in their practices. Frankly, I like both, because science is devoid of emotion and does not rely on anecdote. However, science may not explore every variable and cannot account for the human experience that can incorporate the nuances of detail that a busy breast augmentation practice includes.
So with that lengthy prelude out of the way, I would consider that your own tissue does indeed provide adequate coverage for implants place above the muscle, likely why your surgeon chose this rather than the below-the-muscle position utilized for your girlfriends. Perhaps they chose saline over silicone (not an error, but I and a majority of plastic surgeons have found that there is less rippling with silicone, a more natural "feel", and no chance of deflation with silicone). Silicone is still utilized by a majority of plastic surgeons world-wide for elective breast augmentation. The latest generation of silicone gel implants are cohesive and cannot leak (even if they were somehow ruptured--say,if you were in a car wreck). Perhaps your friends had too little breast tissue of their own, requiring submuscular placement--saline implants are usually placed below the muscle to diminish the visible rippling or "edge" feel, can leak and deflate, and usually feel less natural.
Going below the muscle taught most of us that capsular contracture was less common in this location (at least for those of us who are older and saw lots of years of high CC rates) because of the decreased contact with breast ductal bacteria that happened above the muscle--where the exposed breast ducts could "bathe" the implant surface with bacteria. This is why, even when the FDA re-allowed the use of silicone gel implants (2006) for cosmetic patients, most plastic surgeons still placed silicone gel implants below the muscle. Less capsular contracture is a big deal, especially if you are unfortunate enough to have this happen.
So, regardless or type (silicone or saline) implant you and your surgeon choose, going above the muscle may be OK because of your own tissue coverage, but NOT OK (IMHO) because of the increased risk of CC, even with antiseptic or Betadine irrigation. Silicone textured implants were designed to reduce the incidence of CC with above-the-muscle implants (before the FDA restrictions of 1991), and depending on the age of your plastic surgeon, may be something he is not familiar with. More recently we have the "gummy bear" form-stable McGhan 410 investigative cohesive silicone textured implants. I agree that textured implants have no place below the muscle, since they cannot move and their bacteria-isolating and collagen vector force-disrupting effects are not needed (as they were above the muscle). If your surgeon "NEVER" uses textured implants, that is unfortunate, becuase there are a few situations where years of experience with thousands of patients may make this not only a reasonable choice, but a "best" choice. I RARELY use textured implants, but would certainly do so in select cases if I were contemplating an above-the-muscle augmentation or revision! So at least that friend is correct about this specific recommendation.
I think your surgeon is choosing the right type (silicone smooth) and the right size (500cc = about two cup size increase) for you, but is choosing above the muscle because you have adequate tissue coverage and this will (probably) work out just fine and with a great result. He is avoiding below the muscle concerns (activation distortion, sometimes more pain or bleeding risk--though this is more surgeon and technique-related, and slightly less ability to deal with breast droop or sag), but balancing this are the increased risks of capsular contracture (low with triple antibiotic or Betadine irrigation, but still higher than below the muscle) and a slightly less natural slope below the collar bone (better with below the muscle placement.
So, as other of my colleagues have said, there is no RIGHT or WRONG, just a selection of choices based on science, experience, and your own anatomy and preferences. DO NOT rely on friends as much as your plastic surgeon. I would recommend asking him for a discussion of above or below the muscle, what technique your surgeon uses for each (blunt dissection vs. careful cautery control of bleeders), use of Keller funnel (no-touch technique) or not, and whether or not he is comfortable with your choosing below the muscle. If not, get a couple of more consultations with ABPS-certified plastic surgeons with years of experience in cosmetic breast surgery for additional opinions. Your own plastic surgeon seems to be (mostly) on the right track. Your friends are (mostly) right also.
I would recommend 500cc silicone gel implants below the muscle, choosing the profile based on your breast base width (pocket diameter). For more imofrmation click on the link below. Best wishes!