this thread is probably better posted in the DL forum, but alas. DL is much better operationally and from a product perspective than UA. UA is much better for mileage redemption, upgrades, and the like than DL (but that is changing).
i tend to enjoy my experiences on DL significantly more than my experiences on UA. i have had bad experiences on both airlines, though admittedly the last bad DL experience I had was several years ago while I have had several bad UA experiences in the last year alone. that being said, i have flown a lot more UA than DL until just recently.