My lovely bride took Amtrak from Chicago to Florida one year for Spring Break. She hated it. Are you comparing a sleeper berth or upgraded accomodations to airfare, or will you be riding the regular coaches?
Be prepared for many stops, etc.
It is kind of fun for the people you will meet. She said they went through all the areas she wanted to see at night and scummy cities during the day. That of course may be different.
Bring some good books, earplugs, snacks, and have fun. It will be an adventure.