I am a bit of an expert with this..have had saline unders / anatomical gel overs and now round overs (yes finally happy and no problems..phew)
My advise. Go see at least 3 PS. Take pics of what you consider perfect/too small/too big. Pics speak louder than words. As far as placement, I would say overs are the best IMO..unders you get distortion when you use the muscle. Also there is not a lot of natural tissue under the muscle ..so why would you put an implant there if you don't have to
I would also go with silicone. the difference in feel between saline and gel is like night and day..the gel just feel a LOT more natural. The new ones can be cut clean in half and still not leak so they are pretty dam safe. We have been using them here in Australia since 1992..
If you have any specific questions..feel free to PM me. Oh and get some Arnica..is great to take straight after BA as helps with healing