I have used both of those and expedia.com. Each may offer different specials depending on when/where you are going and if you want to add a rental car or hotel to the trip. Go to any of those sites and fill in the info for the flight (departure, destination, dates, people traveling). A page will come up with the various options. Pick one you like and go from there. In some cases you can pick your seat at that time. I typically print out the recipt that they email me. When you show up at the airport, you go to the carrier and use the machine to check in or talk with one of the agents. The last bunch of times that I have flown I just had one small carry on bag. Makes everything easier (they'll likely charge you extra at the gate if you check luggage). If you use the machine to check in, you'll swipe the same credit card that you used to buy the tickets and it will print out your passes. Note that it has been taking longer to go through security and they are always changing what you can and cannot take on a flight.
Andrew