freakmonster...don't know your GF's situation but usually the doctor will discuss the options with you and make his recommendation. There is no absoute rule. There are a ton of sites with info on this if you do a little web searching. In general though, there seems to be more consensus that under the muscle is better (ablility to breast feed, less hardening of breasts due to scar tissue, better overall appearance, etc.) Just make sure you have a good doctor and that your GF's expectations are realistic. Some of the worst results are because some women try to go very large for the amount of natural breast tissue they have. This results in very tight, hard , unnatural looking breasts. Good luck.