Butt augmentation has become very popular over the past several years. It has been the fastest growing aesthetic procedure in the U.S. the past few years. There are two methods for butt augmentation. The first is the Brazilian Butt Lift (fat transfer) the second is Butt implants, both are good options and just dependent on the individual patient.
Butt Implants are great for thin patients that do not have enough fat for a Brazilian Butt Lift or for patients that are looking for a lot of projection. The implants can be done alone or in combination with fat transfer.
There are several ways to place the butt implants and the methods vary from surgeon to surgeon. I commonly place the implants in a subfascial position for several reasons. One, the subfacial plane is very clean and avoids damaging the muscle or nerves. However it still holds the implant in the proper position and decreases the chance of scar tissue forming around the implant. The fascia is the strong covering that wraps the muscle, that is why it can hold the implant in place. The submuscular plane can also be used but in my experience, patients tend to have more pain and over time has little benefit over the subfacial system.
In terms of the implant both the round and shape are good implants. I personally use the round implant for several reasons. It gives good projection, which patients that receive the implant are typically looking for from there procedure. Also the butt take abuse in our daily life from falls and so forth, with the round implant you are less likely to have problems if the implant shifts at all. With a shaped implant if it moves at all it will need to be revised, this is not necessarily true with a round implant.
I recommend that you see a board certified plastic surgeon about your butt augmentation that performs a high volume of these procedures.