Treadmill vs Road: Which makes for the harder running?

AJ submitted a question for us to answer on the Ask Coach Parry podcast, and she wants to know about treadmill vs road running. This is her question: “I run a fair bit on the treadmill and often struggle to get ‘going’. Can you tell me which is better for you treadmill vs road running and why, and also why does treadmill running seem so much harder?