I alternate between OWB and IWB depending on weather/season and how clothing choice will affect concealment, including with a P239 compact and SP2022 full-size. I prefer leather for IWB for comfort reasons, but kydex for OWB when there's no direct skin contact. My Sig-branded holsters use "paddle" mounts, which are adequate, but a well-constructed kydex holster with belt loops is (in my opinion) better for concealment. I'm of fairly slender/average build, so I won't claim to have quite the same holster needs.
Being that a P320 is striker-fired, you won't run into some of the issues I've had with my Sigs, which is the hammer's tendency to dig into my skin if a holster doesn't provide coverage there. I currently have a Stealth Operator for my P239 that has worked well so far, and an R&R kydex for a Steyr M9 that is fantastic.