There are many factors to consider. You really should discuss options with a plastic surgeon who is very experienced in breast augmentation. Someone who uses all types of implants from different manufacturers; is able to place the implant in all the various planes, including subfascially; and is able to insert the implant from the armpit, under the breast, and through the areola. The decision on what type, where to place the implant, and how to insert it will depend largely on your anatomy and the result you want to achieve.
For example, if you are thin with little natural breast growth, submuscular placement with a shaped highly cohesive silicone implant is going to give a great natural looking breast. You'll need the muscle tissue to cover the implant so that it looks nicely contoured. But shaped implants are limited in size (Mentor's largest is 755 cc for their tall, high profile teardrop implant). That may not get you to a D cup if you have little natural breast tissue and a 40" chest. Also, these "gummy bear" implants need a larger incision and are almost always inserted inframammary (breast crease).
You could go with a non-shaped silicone or saline implant submuscular, but without much breast tissue, the shape is not likely to look natural, and animation effects are going to be more obvious. I think it's best to be on HRT for 2 years before doing breast augmentation, then size the implant based on your existing breast width and height. Otherwise, don't be surprised if your breasts look fake. Now if that doesn't matter to you, then you have far fewer restrictions to consider. Good luck.