Smooth or textured implants for over the muscle BA? How likely is it for smaller implants (300-350cc) to slip out of the pocket?

In general I prefer smooth implant placed behind the muscle unless there is a specific reason to choose textured implants or a subglandular pocket.  Different surgeons have different approaches and I don't think there is one right answer, but in my experience for most patients this approach can produce a very nice result.

Hello and thank you for your question.  The best advice you can
receive is from an in-person consultation.  I prefer smooth silicone implants over textured.  However, I also prefer under the muscle.  There are several advantages and disadvantages to over vs under the
muscle.  Under the muscle has the advantage of lower capsular contracture
rates, less risk of rippling, and better visualization with future mammograms. 
Most patients say that under the muscle also has a more natural look and
feel.  The size, shape, and profile of the implant is based on your
desired breast size/shape, your chest wall measurements, and soft tissue
quality.  This decision should be based on a
detailed discussion with equal input from both you and your
surgeon.  This entire surgery can be
performed with a small incision technique.  Make sure you
specifically look at before and after pictures of real patients who have
had this surgery performed by your surgeon and evaluate their results.   The most important aspect is to find a
surgeon you are comfortable with. I recommend that you seek consultation with a
qualified board-certified plastic surgeon who can evaluate you in person.
Best wishes and good luck.
Richard G. Reish, M.D.
Harvard-trained plastic surgeon