is there much advantage to once fired,neck sized over brand new brass i have always used once fired

Is there MUCH advantage to once fired brass? Depends. I say yes. Once fired brass will raise pressures and shoot a bit faster than new brass. Some of the energy in new brass is absorbed by the expansion of the brass. If you reload it's likely all except the brass used in the initial fireforming will be a bit faster and have higher pressures for the same weight of propellant so it's important to know from the beginning where the optimum load will be for the brass you will be using. I fireform with Cream O' Wheat, a fast pistol powder and a wad of toilet paper to eliminate the variance between new brass and fired brass.

There's an advantage to once fired brass if the firing was in the gun you intend to load for. Otherwise, once fired brass needs to be FL sized. I loaded some once fired brass given to me by a friend with the same kind of Mark X 30-06 as mine. I just used my neck sizing die and loaded it up. It wouldn't chamber in my rifle unless a full grown gorilla was operating the bolt. In his, it was fine. Any brass I load for a particular gun gets FL sized unless it was fired in that gun. I have tried mixing FL sized and neck sized rounds at bench sessions, and much to my surprise, The groups were about as good as with carefully loaded neck sized rounds. My only reason for neck sizing is that it works the brass less and I don't have to lube the whole case, making cleanup easier.
I fire-form using a full load, albeit with super cheap bullets. IMO, it is better to form that way. Besides, I can always use the practice for position shooting. Since I don't have a bench out in the field, the more rounds I fire in practice, the better field shot I become.