Ah-HAH! I found the torsional rigidity stats I was looking for:
BMW E46 Sedan (w/o folding seats) 18,000 Nm/deg
BMW E46 Sedan (w/folding seats) 13,000 Nm/deg
BMW E46 Coupe (w/folding seats) 12,500 Nm/deg
(couldn't find coupe w/o folding seats, though . . . )
So, all else equal, a 300i is a bit more structurally rigid than a 330Ci. ZHP sedan FTW!