To the OP,
Recently looked at the same thing for an upcoming purchase. I am opting for in/on walls. Compare Axiom's specs between in/on wall versus on wall and you will see the in/onwall covers a slightly larger range of freq.

Mark
Pio 94,SB3,BD30,A35,Pio 48AV.Pronto 7000 controlling all. Getting ready for 22s, 150 & QS8s.