Hey hun, it's a really hard decision. Both types have pros and cons. Anatomical may lead to rotation, but if you follow your surgeon's instructions to the letter, the risk is minimal. Rounds are a bit more likely to cause capsular contracture, but not for everyone. Anatomicals look more natural, but many rounds these days appear to settle into a natural shape too. For me, anatomicals and under the muscle were recommended due to my lack of breast tissue and my desire for a totally natural look and it was absolutely the right choice - for me.
Over the muscle heals faster and drops and fluffs faster.
Maybe you should make a list of the pros and cons and then decide which one has the most pros for you - good luck with your decision and I'm sure other girls will chime in with advice and preferences and it's all helpful to get everyone's experience xo