OK I finally cam post about something I have experience with
I have pretty bad ptsosis (sagging) and loss of breast tissue from working out, losing weight, etc. I saw the surgeon, and he recommended implants combined with a modified mastopexy (donut around the nipple) lift. I went with silicone and over the muscle. I weighed the advantages of over the muscle vs. under and decided to go with over because of the unnatural look when working out (my sister has them under) and because of the recovery time. I also read a lot on fitness and implant forums, and many women had gone over the muscle and were happy with it.
I also wanted to be a full c small D, so when he got in there he decided 400 cc would fill up the skin and look good -- I had been a B cup.
They looked gorgeous for about 3 weeks, then fell... My only option was to have a full lift, which was my second surgery. I had it done about 3.5 months after the initial. In hindsight I should have gone under the muscle then but I didn't really want to for the same reasons as before, and I didn't have the time to take off work (I'm a pilot).
In about 2 months, they fell again. They do look much better than before the first sugery, BUT they have quite a bit of sag. The only saving grace is that the nipple is pretty high, so they don't look as bad as after the first sugery.
I now need a third surgery to put them under the muscle and have another full lift. Needless to say I am dreading it, and have put it off already for more than 2 years.
So my advice is to go under the muscle in the first place. Good luck!