Will, you miss my point.
You're right, there could have been games that we may not have won without James in the side. But James isn't going to come onto the pitch with us 2 goals down with little time remaining and score a hat-trick to win the game. An extreme example I know, but that's what I mean by a match winner.
Replace James with a lesser player, say Evans, and he will still provide a function that will enable the team to play its normal way. Replace Powell with a lesser player, say Windass, and he won't win us too many games with a piece of genius from nowhere.
So that's why I'd go for the matchwinner.