This is what I know from personal experience: United charges more if you do not print a boarding pass online. It cost only $11 for the flight to Grand Teton National Park but for the return home, we paid $15 per bag because Yellowstone does not have the Internet. So if you are going to end your trip where accessing the Internet is impossible, don't go with United. I don't like that airline. It charged me for the first bag.
If you are traveling to another U.S. city, you can't beat Southwest. That is the only one, unfortunately, that never charges for the first or second bag. The only downside: they're not able to fly outside the United States.