Peachey is a liability, Salmon is not.Peach is a better utility than Salmon is full stop. Salmon is a better defender and perhaps a little better as an out and out forward in terms of hit ups. Probably better at being a 'controlling half' as well.
But Peachey gives far better cover at centre - can slot in at #6 and do a decent job and can also play hooker at a pinch.
Peachey is also a far more dynamic runner of the ball in any position.