Clean update/install of new Android version (non-nexus)
I don't think a factory reset would revert your device back to 4.4, I'm pretty sure it would still be on 6.0. A factory reset would wipe all user data on your device without touching the OS.

If you some want to wipe all installed apps (bar system apps) and user data then a factory reset would do. If you want to wipe the OS too then you would need to find a full device image.