The flights for me are in the ballpark with the rest though some times I pick UAL on cost or direct vs. not direct. I will also pick flights and times based on aircraft. 757 trumps 737 trumps MD88, etc..
All things being close I choose Delta as I like the experience more than I like UAL experience. A couple hundred dollars to be more comfortable is always worth it to me. I also have the Amex Plat card so Sky Clubs when on Delta.
Background.
Travel is CONUS so far, I buy first class tickets A and G mostly. My flights generally start out in Orlando. I only buy FC tickets for health and comfort reasons. Compressed disks, cracked disks, little claustrophobic, and generally not fond of the public / people.
Last week was MCO to IAD chose UAL to get direct, was not impressed gates where crazy compared to Delta gates at IAD. When I go back in a few weeks might do Delta again and connect.
Sunday is MCO to DEN chose UAL to get direct and most delta connections too much time on a MD90.
Will be doing MCO to LAX soon and will go Delta 757.
Doing family vacation MCO to SLC in a bit and will go Delta. Might send my kids Jet Blue though...