Well, you'll be making a big mistake if being ontime is important to you. You were just incredibly unlucky. Even with the rare ACARS difficulty UA had yesterday, they cancelled relatively few flights. It looks to be about 20 flights at most. That speaks to good performance, not bad. You were far more likely to be cancelled on DL yesterday with their weather issues. And many of those cancellations came last minute after long rolling delays. That's poor customer service.

The stats say that UA and DL are very close in ontime and cancellation performance. In fact, UA is probably a better operation for ontime when adjusted for the more congested airports they hub out of. Take EWR/LGA for example, UA mainline at EWR beat DL mainline at both EWR and LGA in ontime percentage (EWR is more punctual in genral than LGA, no matter the airline). If you add a connection to take DL instead, statistically you're much more likely to experience a delay when adding a flight, and you add the chance of a messy misconnect too. There is no comparing a connection from LGA versus a nonstop at EWR. The right choice for being ontime is EWR.