Let me start out by saying that having the top 6" of your pool exposed with the concrete butted up to it isn't even an option these days. If your pool contractor is telling you that is an option, start talking to different inground pool builders who are more experienced!
The pool top will start to fade and it gives no additional strength to the pool . . . just not the correct way to do it.
Concrete and Strength
By pouring the cantilevered concrete on your fiberglass pool, it covers the top lip of the pool which is generally scuffed in translate and locks your pool in place. You should have concrete under the lip and then on top of the pool in one solid pour to encase the pool. The amount of concrete under the lip of the pool varies on between pool contractors, however is generally 6"+
Many inground pool builders will put rebar in the top lip of the fiberglass pool and extend it back 3'. This locks the top of the pool & rebar in concrete PLUS 3' of the pool deck. What that does is make your pool 3' bigger around the entire pool as it is one solid unit.
Why is Cantilevered Concrete on a Fiberglass Pool so Important?
Strength, Strength, Strength. By pour cantilevered concrete correctly is will give you pool a cleaner finish, look, and lock down the entire pool giving it exceptional strength.
The fiberglass pool cost will be more as not ever concrete guy can pour cantilevered concrete. It is more difficult and time consuming but that explanation is for another time.