My tie breaker is usually 1) if it's a direct flight or not 2) ease of upgrade (even to econ premium)...I could save myself over $100 (that was as of yesterday) if I fly United at the end of March vs. Delta, which prompted me to ask in the first place. About a 4-5 hour flight but would be early AM or evening so in reality I'm likely to fall asleep + Netflix on iPad.
I've done plenty of redeyes and both were fine...econ/premium econ on the Delta bird was more comfortable. Business class on United was worth the $200 I paid to upgrade to ensure I could sleep comfortably.