Can't really go wrong with either right now. I'm at Delta, but I think right now United has more upside (unless you want to live in a DL base then 100% choose Delta). I think UA will have a consistent quick upgrade in both SFO and EWR whereas Delta's is streaky. Also Delta has hired far more pilots already during this wave. United has more access to widebody flying if that's your thing. Delta's profit sharing is awesome, but United has some better contractual things also and profit sharing isn't a guarantee.
All in all, if bases matter choose accordingly. If upgrade/seniority matters most then pick United (or widebody flying).