Generally speaking, the seats are much more comfortable, especially if you're my height (6'1). I've found the seats much more comfortable than the airline seats, unless you're attempting to sleep overnight - then, a sleeper is much more preferable.
On your trip, you will encou8nter a cross-section of American society, and the atmosphere is more social than any trip you've ever taken by air. Just remember that if you're polite to others, they'll probably reciprocate for the most part. Other things to remeber: Amtrak doesn't "own" most of its tracks, they're owned by the private fireight RRs. The fireight railroads also do their own traffic control, which they have to pay entirly out of their own pockets. So, some railroads will give preference to their own trains over Amtrak.
What train will you be on? If it is one of the long distance western trains, that can also be a factor in the comfort levels, depending upon the equipment assigned.
Other than that, best wishes on your first trip