I prefer Fultz because of his size and scoring creation. Lonzo Ball is taller but I think long term Fultz is more likely to have the girth to defend multiple positions. Ball is great at finding open shooters, but I think the Celtics need someone other than IT to get to the hoop more than a guy like Ball who doesn't break his man down off the dribble as often.
Long term, if we draft either I'd expect us to develop them slowly with the hope that they are ready to run things by the time IT's contract is up. Short term, I think we would play them as the back up point.