I would say the convenience it’s highly dependent on the destination. Here the train station it’s often near the city center while to get to the airport you need additional transportation, As you said you have more legroom but also there is enough space to use a laptop and actually work connected to Wi-Fi. I’m not limited in what and how much I can carry with me, and the train is much quieter than a plane. And finally, the train is much less stressful and traveling time doesn’t really matter when is the company’s time.
I would say that if you don’t have an airport in a 30km radius from the destination, the train still wins around until 8hours trips. Above, I would consider travelling with the airplane.