Are there differences? Of course. But it is not like choosing UA First vs. SQ First for long-haul. Cold food on DL, hot on UA and I think AA. No IFE on DL or AA, audio (incl. Ch9) on UA, Nicer Marine Air Terminal for DL and CR at ORD while AA & UA have clubs on both ends. Nothing too major in here to really drive a choice.

Go with whichever carrier you are trying to accrue miles on and/or get status on.