I switched AA to UA (from AA Plat) 15 years ago for exactly the reasons you’ve indicated. At the time UA has the best route coverage for me ex-PDX so that’s who I went to. I didn’t bother with a status match, but my flying increased significantly and I had PremExec quickly and 1K by end October.

And UA has improved significantly in the last 2 years. Their operational performance is close to DL and on some metrics neck and neck. While I don’t think they’ll overtake DL (DL is also improving) I don’t think you’ll be disappointed with UA.