Id rather we keep Butler and lose Castagna or Baker instead
Baker looks to have a lot of upside IMO. His decision making is that of a new player, and once he gets that down, he'll leave the other two well behind I suspect.
George is a very good tackler. He is one of the best front on tacklers in the league I reckon. He sticks them, and it isn't something a lot of players are that good at.
I think both Baker and Castagna could also run through the midfield if we needed them to, and while not being superstars they could make a contest. I don't think Butler can do that, certainly not at the moment anyway.
Butler just doesn't look the goods compared to these two.