You will definitely be higher on the F UG list as an ATL based UA flyer instead of an ATL based DL flyer. UA also tends to have more E+ seats on its planes compared to DL, so you should be able to consistently secure an extra legroom seat. The one big downside is the lack of nonstop options compared to DL out of ATL. However, if BFL is your primary destination, UA probably would make more sense because ATL-DEN/SFO-BFL is a lot better than ATL-LAX + bus.