How do i decide whether i need to put the app in system/app or data/app?

[anand devarajan]

Sorry for the long explanation,
Before the actual question, let me explain my understanding about app permissions and directories
I know there are different directories in android for apk installation.
system/app
system/priv-app
system/vendor/app
data/app

Out of these,
system/priv-app has more privilege(systemorsignature level permission allowed) containing trusted apps(but not sharing the same signature with platform) and platform signed apps
system/vendor/app is for vendor specific applications controlled by vendor flag(no system level permissions)
system/app is for 3rd party preload applications bundled with system image(no system level permissions)
data/app id for 3rd aprty apps(no system level permissions)

Any app installed in system/* cannot be uninstalled by user.

So if a 3rd party app is installed in system/app and data/app. will there be any differences in behavior, access, privileges, functionality and rights(like stuff that only system/app can do and not data/app or vice-versa)?
what more can i achieve by placing my app in system/app that having in data/app?
How do i decide whether i need to put the app in system/app or data/app?