Forget, for a moment, who moved the ball more. I can't speak for Goughy but for me, the amount of swing/cut/seam a bowler gets is just one consideration in rating McGrath above Donald. For me, it's also about how they used the conditions they had available, how they reacted to being hit around, who they got out and how they did it, how they targeted them, their tactics against players, at what point in a game they took their wickets, etc. Intangibles.
One example; McGrath and Donald were both brilliant front-runners but I think McGrath was better at pulling back when the batters were on top whereas my perception was that Donald was more likely to drop his bundle, even a little. I also felt that McGrath was better at targeting players' weaknesses and not letting up and had a better plan B if things weren't going well for the team. Now, no amount of poring over their records or scorecard reading is going to be instructive to that level but it's reasons like that why I rate McGrath a touch higher as an all-round quickie.