How many segments exactly?
If you fly over 60 segments on UA you would get Gold - and as you prioritize upgrades over lounge access it is better for you. Try to match your delta silver/sign up for challenge just before your flights with United will start.
If it is under 60 segments I would go for A3 gold - you would get free bag just after 4 segments in Y booking class/6 segments in any other flexible classes, gold after 26/38 segments (which is 3 free bags and lounge access while flying domestic)