The Dr will advise whats best for you on the unders/overs front, I personally would always go with overs if given the choice as I have had had both unders and overs, I find the overs far more comfortable. The recovery from unders was hard, plus I always knew they were there if that makes sense? You always have some slight tension in the muscle which can pull tight against the implant and it was really unpleasent. My overs feel like a part of me, I don’t notice them at all, tyhe recovery was a walk in the park and I was off pain meds after 2 days. Size wise, the difference between the two is really minimal, less than a cupsize so either size should yield a similar result.