I do believe DL is better on a level playing field for sure and all things being equal I'd go with DL, but if I was local to a UA hub I would probably give them more than one chance.

In fact, because I have a lot of dealings near Dulles, I keep giving UA shots simply out of convenience. I've been burned by UAX cancellations before, but I've also been burned by DL delays which is especially frustrating if you are taking a detour via DTW or ATL.

The scenario in the OP seems like it could have played out the exact opposite way too if Delta had had a systems outage, which of course happened not too long ago. DL in this case simply sold you an expensive ticket. I think UA could probably have done that in August too.