I believe the most comfortable way to travel is by train. You have conveniences such as spacious washrooms, food/beverages, comfortable seats and some terrific views along the way. We have traveled in Canada by the VIA Rail system. I can say that the two times we have gone from Windsor to Toronto have been easier than if we went by bus. We did use the GO Rail system to travel to Niagara Falls from Toronto. We did take the bus on our last trip from Chattanooga to Detroit, and that was an adventure I recommend to anyone. So I am going to say trains if you have time to get from place to place.
I would think that going by train or bus would be a great way to see the country. You can’t see much from the sky. I have done cross country in a minivan when I was a teenager when my family went to California and then on a separate vacation, we went up north to New York City. I am not a good long-distance driver so I would not want to take a car. Also, there would be a lot to look at.