A consideration for the times you are unable to upgrade (which may be hard as a Premier out of key business destinations)-- EconomyPlus, which is an absolute lifesaver on longer flights. I have really gotten used to it over the past few months (am usually on 757s), but have recently been on a few flights without it. Let me tell you-- I was in an aisle seat and as soon as the person in front of me on the 747-400 with 31" of seat pitch reclined all the way, I wanted to get off that plane.
Frankly, I have never flown Delta and have no idea what your chances of upgrades are.
But in defense of UA's on-time record, I would suspect that it has a lot to do with where their hubs are located. Atlanta doesn't often get too much inclement weather. Chicago, Washington D.C., and San Francisco are very prone. And from Newark, you probably are too (heard that CO's operations were crazy this weekend-- had a friend who was going to PHL via EWR and never made it!). In the past year I have been on only one UA flight that was more than 15 minutes delayed for non-weather problems.
Some other considerations: Starbucks Mocha Frappucino. Can't live without it! Also, word is that UA tends to have better Y catering.
[This message has been edited by dw (edited 02-20-2000).]