Johnston has played 359 mins of league football this season (1,172 mins in all comps). Ralston has played 789 mins (987 mins in all comps)
Johnston has featured in 14 league games, Ralston has featured in 13.
Ralston has 8 caps already, he's 100% ahead of Johnston on the pecking order. Might be different if Johnston was starting most weeks but he's mainly a sub for league games.