I am GGL and UA Global services - I live in Scotland and had UA 1k for a long time before getting global services and also deciding to go for BA gold.

UA IT is in general miles ahead of BAs, as you say the app and instant flight credit is a very nice feature compared to BA where it takes 3 days for flights to credit.

Polaris seats in my view are excellent and the Polaris lounges are great. The access policy for them makes it much less crowded as it's no guests and J only. I always eat in there because the food onboard is generally not great. It also lets me get a good sleep on the flight home. Domestic first and transcon are ok - again better seats than club Europe but that's about it. Food offering again is a bit meh.

BA onboard service is generally a lot better than UA in my view. The GGL line also is on a par with GS for dealing with any irrops.

I will likely hit 1m miles with UA some point next year which will get me lifetime gold (star A lounge access etc), after which I'll focus on trying to get BA lifetime gold and probably just maintain 1k - im very fortunate I get J by work travel policy so I'm not in the fight for upgrades as mentioned above, those lists are extremely long in many cases post COVID - as a GS I use my plus points to upgrade my team members when they are on long haul ans a GS sponsored upgrade always trumps a 1k as they are processed from a different bucket.

If you can, maintaining 2 status levels is a useful thing if you can do it, gives you options but unless you are doing a huge amount of travel, UA gold is probably the sweet spot and if US based, just pay for UA club access with an annual pass. Gold gets you economy plus at booking which is a decent benefit if you are not flying paid J.