I started full time travel again in September and did ~65K miles this year, 59 on united and 6 on Delta. Delta's customer service has noticeably gone to hell since I was platinum. Their FF program is more difficult to reach the highest level than United's program. As a Gold on United, I consistently get more upgrades than my colleagues who are Diamond on Delta. Their phone App is useless and their website is not much better. The only advantage I have seen is that lounge access is easier. I travel international weekly, so I get that anyway.
FWIW, I started getting First Class upgrades on United at around 20K miles. I think their system may have an algorithm to detect long term flight patterns of frequent flyers.