I would also recommend UA for many of the same reasons mentioned here, but also because it's simply a far more professional airline than DL. In the event that you occasionally have to fly DL, you can always earn Mileage PlusŪ miles on DL domestic routes. These flights unfortunately still do not count towards base miles, but it could be helpful if you're interested in sticking with one program.
The only other consideration for DL could be if you were paying for tickets yourself, as the AMEX SkyMilesŪ Gold and Platinum CC's offer double miles per dollar charged for DL flights. On the bright side for UA, at least you can buy relatively inexpensive upgrades (around $31 or so?) that you can use if space is available. With DL you can run the risk of paying a K or higher fare for the privilege of being eligible to upgrade--only to find out that none are available. I don't know what the market is like in EWR, but here DL has L fares available on every competitive route--often requiring no Sat night stay and little advance--so they're not just restricted to "leisure travel."