Unders tend to be for slimmer girls to achieve a more natural look (more of a natural slope) then are also meant to be more benficial for the long term and the muscle helps support the implant.
Overs tend to be for women who have a little more skin, so you are more likely to be offered these if youve had sagging due to having children etc as it fills out the excess skin to give a fuller look.
He will offer you what he thinks is best and singh does bigger boob jobs, hes the big boobie man
and he will give u a choice of 2 or 3 implant sizes and you should be able to try them on.
Normally the surgeon will advise you on what he thinks is best, rarely you will have a choice 
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