I'm late to the game for this topic, but I'd have to argue that 'P' is a better choice. More traditionally with PC games, 'P' will pause a game while 'ESC' will bring up some sort of menu. Granted, pulling a context menu of some sort will achieve the same purpose of stopping the game momentarily. I think it's mostly semantic. Players will traditionally hit 'ESC' because they want to get into the menu and it's become a norm. But 'P' typically performs a single pause state without extra stuff, like a menu.
When in doubt, and for the sake of making life easy, I would recommend having both keys do the exact same thing and listen for both.