The difference between overs and unders is only really dependent on how much breast tissue you have – if you don’t have much they usually suggest under the muscle. Unders are supposed to suffer less from complications like rippling and look more natural, but with overs you usually can go a little bigger. I have seen both overs and unders look lovely and natural too 
High profile are usually used on smaller framed women, they have a smaller base than other implant types and so project out from the chest more. They can also allow you to go a little bigger than the other types if your frame isn’t very big. Have a read here – http://www.breast-plastic-surgery.org/high-profile-breast-implants.html