Hi Sophie,
what the surgeon said to me is.. Unders are used for people with no breast tissue to cover the implant.. the Americans like unders cos they create the look where you have quite a gap in between (not for everyone tho ) he said overs create more of a cleavage and look the most natural… as for the sagging quicker, all depends on how you look after them, due to saggin having them place either under or over doesnt matter regarding sagging ( i dont think)
xx