I am UA 1P and Delta GM. To me, the only advantage that Delta has over UA is much more frequent first-class upgrades. UA has E+, and, more importantly, you earn miles that are actually worth something.

I have a ton of Skypesos sitting in my account and have never been able to use them for anything approaching a decent value. On UA, I've used miles for saver awards to Australia, Europe and South America and, with a little flexibility, haven't had too much of a problem finding dates that work for me.

I find the service about the same on both airlines.