I agree with those who write there is no one "worst" airline, because every carrier has its pros and cons, and suits the needs of some travelers better than others.

As fas as DL is concerned, I think whether or not DL is better or worse than UA (or any other airline) is largely a matter of personal preference and experience.

As far as E+, while UA remains the industry leader in this category, it won't be for long, because DL already has an E+-style section called "Economy Comfort" and is in the process of installing it on all its mainline planes, and its larger RJ's (similar to UA), and AA has announced it is adding an E+-style section.

As far as FA's, I fly a lot of both DL and UA (mostly the sCO flights out of EWR), and I have found DL's FA's on average far friendlier and less snappy.

Also, as far as upgrades, the general consensus among NYC-based fliers, is that elite upgrades are much easier on DL than on sCO (UA), if nothing else, because DL's tends to fly more planes with larger FC cabins than sCO does.

But again, it's all a matter of YMMV.