For me, the fact that DL's operations are vastly superior to UAs (my perception, but many here seem to feel the same way), and the fact that I could hit top tier Elite with my credit card waiver (as has been mentioned already, UA does not allow this), are the reasons that I have chosen to do my flying with Delta.

However, I miss the *A partner flying on FF miles. Definitely a superior group of airlines overall. But DL is superior to UA in the US.

As many mention, it's all about how you fly. If you are a domestic road warrior (or "semi-road warrior"); then Delta is operationally superior to UA.

Spending your year dealing with delays, cancellations, constant flight changes; it's just not worth it, to get a "potentially" better redemption for an individual (or two) vacation trips.

Now, if you are an in-frequent flyer relying on leisure trips + credit card miles/transfers and promotions; it's a different scenario.

For those who fly a lot, in a general sense, Delta treats them well. That is of more value to me, personally, than a few (potentially) superior redemptions. And, I can do those redemptions anyway (based on credit cards, Membership Rewards) and selectively use my SkyMiles when the opportunities present themselves.