Hi Josilyn,
Getting the size right is very important, and worth taking the time to get it right. Remember that cup size is not a good way to measure size. The volume of a 34C cup is a lot less than the volume of a 40C cup. What you want is a BA results that provides good proportion for your unique body shape and size. I really recommend that you have 3D virtual imaging done before you decide on a product type (saline, gel, cohesive gel) and size. Most reputable breast augmentation surgeons have the equipment to do a 3D simulation. My surgeon used a Vectra 3D imaging system to help us decide on the most appropriate size for the look I was hoping to achieve. Keep in mind that the broader your chest, the larger the implants need to be to achieve good physical proportion, and the harder it will be able to create cleavage. Placing the implants closer to the breast bone is not advised because it results in the nipples pointing to the sides instead of straight ahead.
The cohesive gels are nice, but they are rather firm compared to the silicone gels, and a lot firmer than saline. I chose the highly cohesive (gummy bear) implants, but my chest is a fairly manageable 36" band. I'm now a 36D and can achieve cleavage with the right bra, but it is not as easy had I gone with gel or saline. Under the muscle will be more painful because of how much the muscle has to be stretched to insert the implant. The larger the implant the greater the stretch and the greater the post-op pain. I had 440 cc gummies with submuscular placement inserted at the breast fold, and the pain wasn't that bad. But that's just me. Sensitivity to pain is a very individual thing.
I am very happy with the final result, but personally, I feel that FFS and voice training/surgery is more important to achieve passability than BA. One can use breast forms to achieve a more feminine shape, but there's not much you can do to hide masculine facial features, a prominent Adams apple, or a deep voice. BA was the last surgery I elected to have for that reason. Most girls put a very high priority on BA, I realize, so to each her own. Good luck on your surgery, follow the post-op instructions religiously.