Tread depth is only one factor. As tyres age and suffer performance degredation irrespective of tread depth, thus if you have 2 tyres of the same type and tread depth but one is 1 year newer, the newer tyre will have better adhesion.
Best bet with the modern ESP systems seems to be to rotate the tyres to wear out at the same time, and not mix and match tread / age / manufacturer - my car's ESP was a complete pain in the butt when for a month or so I had different tyres front to rear.
I'd stick the new tyres on the rear.