Don't fall for "United is better operationally" hook, line & sinker just yet. Here are a few points:

1. United is struggling to get flights out on time just like DL because their hubs do not have enough workers for below-the-wing functions like getting fuel to the plane, bags loaded, cargo loaded, or catering.
2. Speaking of catering...United is switching catering companies by October. This means the current caterers are basically doing a "work stoppage" and several hubs are having a hard time getting flights catered. IAH is the worst. Several flights are going out with no catering which means you're lucky to get a bottle of water in first class.
3. Upgrades on United are much harder to come by this year. As a 1K I'm batting around 30% (haven't tallied the numbers recently). This is, by far, the worst I have ever done as a 1K, even before the pandemic. And this cannot be blamed on "fewer flights, fewer seats". This summer every United flight I'm on has solicited for volunteers because they are 100% booked.

Fly the airline with the right schedule and price because at United you can forget upgrades (as a Plat, which is what I assume you would be if you did a status challenge/transfer) and will be just as frustrated with delays and cancellations. Yes, I think UA has far better IT than DL though. The app is generally superb.

-RM