Hi Hun, unders are given to women with little breast tissue, they go under the muscle and are a more invasive surgery with a longer healing time. The good things about this is they are now natural looking, sit up high as the muscle acts like an internal bra, less risk of rippling & less chance of capsular contracture & are easier to see on a mammogram.
Over are over the muscle and tissue and is a less painful surgery & you can achieve a more fake look.
Overs can usually achieve more cleavage as they are closer together.
The down side to unders if you are athletic is that they can animate when the pectoral muscle is tensed. This would personally not be a problem for me but it is for others.
I’m having 450 unders on January 20th. Feel very nervous as it will be more painful but other than that I can’t see anything negative about having unders, only positive! Hope this helps x