I am in a situation that is similar yet entirely different.
I too am choosing between UA and DL for 2014, but this is because I am moving from the EWR area to the CVG area. I just reached Platinum on UA and am considering a possible status match to DL. This year I think was an unusual occurrence that I would reach Platinum, if I matched to Delta I would probably reach only Gold by the end of the 90 days (especially because its so hard to reach Platinum on their match program). My travel is mostly business, where I am required to choose the cheapest flight between two points (within reason), but some personal.
Factors to consider:
-DL has more direct flights ex-CVG but you never know when they might cut down on these
-Partner airlines--with US airways leaving Star Alliance after March it will be harder to rig my business flights to earn miles on UA. DL has Alaska and Virgin (I think), UA has Air Canada. Most of my flying is confined to North America.
-Already have 100K lifetime miles with United (after not even two years of traveling with them), and if I have any hopes of getting to Million Miler status I need to be consistent.
-Crossover rewards, Sheraton with DL vs. Marriott with UA. I've been sticking with Hilton and IHG until now because they tend to have better price points.
-Prices for a lot of routes seem to be roughly equal for the two airlines, maybe a little bit less with DL
-My wife has *G through Aegean. This is a bonus in favor of United since we can use the lounge if we travel together. But it's also a bonus in favor of Delta since between the two of us we'd have benefits on two different alliances.
-Mileage redemption is worse with DL, but UA is getting worse as well. On the other hand, DL miles don't expire. I currently have 240K miles with UA and 0 with DL
-My Chase Sapphire preferred points can transfer into UA, but not DL, though it could transfer to DL partner Korean.
Advice is greatly appreciated. I have a flight that I'll probably need to book by 1/2 and I'd like to get the year started off in the right direction.