Judging by your airport preferences, I'm going to venture a guess and say you probably live north of the American Legion Bridge and west of Connecticut Ave, in which case, you probably have a similar flying profile to mine.
I'm curious as to why you consider "IAD" being your closest airport an "issue"?
I can't speak much for DL as I've only flown them once in my lifetime. I used to fly out of BWI on Southwest fairly regularly, mostly because cheap fares, and then I realized the money I saved on airfare, I ended up paying in traffic/tolls/time on I-95/Beltway and/or ICC respectively. Personally, I've found the 20-minute ride home from IAD is way better, especially when stepping off a long international flight or TCON.
Somewhat paradoxically for Amex Plat Lounge access, the Centurion Lounges seem better placed for UA and AA flyers in general (SFO/IAH/DFW/MIA/etc.) and Priority Pass seems better placed for SkyTeam airlines (e.g. many AF lounges are also PP lounges) which would overlap with SkyClub access.
Don't forget to consider your destination(s) as well as your origin(s). If for instance, you're flying a ton of domestic flights to a DL-hub on UA, scheduling and possibly upgrades will be easier on DL. Geographically, I feel UA's hubs are better placed for my preferred travel style (i.e. nonstop preferred, or if connection required, long-TCON flight plus short-hop connection to final destination). The DL hub situation probably means short hop to ATL or JFK, and then long flight to wherever.
You can pay for Economy Plus on UA, same as on DL's Comfort Plus. Price premium is usually in the neighborhood of $40-80 if I remember correctly, though with UA Gold status, you can get it for free for you and a companion at booking. Not sure how that plays out on the DL side.
If you intend to redeem your business trip earned points for aspirational vacations in a faraway land, partner airline availability makes a big difference as well. The Star Alliance network is fantastic for where I'm going (e.g. Southeast Asia, Western Europe) and if you're skilled in the award ticket game, you can get some pretty crazy itineraries with UA's Excursionist Perk -- basically free award flight within a region outside North America, so you could do something like IAD-LHR, stop for a few days, then LHR-FCO for free, then FCO-IAD open jaw return, for the same cost that IAD-FCO would have been.