I suppose it depends what you want from your ruckman.Xerri should get the ruck gig
Gawn averages the same hitouts and marginally better hitouts to advantage.
Gawn averages more disposals and effective disposals.
Gawn averages more than double the marks and contested marks.
Gawn has kicked more goals.
But if you prefer tackles and clearances then Xerri is your man. He averages one more clearance (6 to 5) and more tackles.
As for Gawn Versus Marshall ? Gawn gave him one of the biggest thrashings I've ever seen when two good ruckman went head to head. I can give you the stats if you want.
I got this photo from the Saints board after the game: