Currently for us, Delta is the cheapest! Just under 400 for the 2nd step up, where you can choose your seats. Have to because of the kids. Although last time we flew, they had empty seats at the back and so they let the unassigned seat people sit there and not next to us, so we had an empty seat for the 3 year old to go between mom and dad. I checked spirit, 380 savings hardly seems worth no TV or WiFi for the littles. That all being said, when I booked our Jan 2018 trip, I was shocked, SHOCKED, that I was able to book one way seats from SFO-ATL (visited mom and drove to WDW) and then MCO-SFO, Delta Comfort Plus, for 159 each way. I thought maybe flying had become affordable again! But this year, no so much, those same seats are like 650, so over double.