After being all but Delta-exclusive for the better part of a decade, and a Medallion for 6 years, I spent the last 15 months flying the friendly skies. IMO, there is really not much difference.

Both have good front line people and not so good front line people.
Both offer very limited upgrade opportunities for lower-level elites.
Both offer a very similar hard product in domestic F; although Delta offers a superior product for int'l.
I actually had some pretty good food on United in F/J.

Also, I was at IAD last night during the meltdown. I' sure it was YMMV with gate agents but the folks I interacted with were pleasant enough, even though they were being berated.

If there were two flights to the exact place at the exact time for the exact price, I'd still be more likely to fly delta, because I do think, system-wide, the product is a touch better. But, now that I am going to be a full-time kettle, it's likely I'll be looking at price/schedule/aircraft type before I look at airline.