FWIW, I finally got to fly DL's A220 yesterday from DFW to SLC on my way to Seattle to watch the Rangers obliterate the Mariners.
DL hit a home run with this bird; I was not only impressed with the seats in main cabin compared to AA's Oasis scheme (yeah, I know, it's a low bar to reach), but the IFE screens (which appear to be larger and offer a couple more entertainment options than the IFEs on DL's 738/739). Don't ask me about the lavatories, since I get very nervous about using them in-flight (I tend to, um, go before I board, but your mileage may vary), so I didn't need to use them during my flight. BTW, DL's older 738s aren't that much better than the pre-Oasis AA 738s, in my opinion.
The bad news was that the scheduled A220 on this morning's flight from DTW to DFW (I ended up catching a red eye from SEA to DTW to return to DFW) was swapped out with an ex-FL 712 (at least it wasn't a 320) with no in-seat IFE. The Comfort+ seats in the 712 are much better than the Main Cabin Plus seats on AA, that's for sure, and at least I got to cross another aircraft off my "list" in the 712.
Now that I've had the chance to check out AA and DL for myself, I'd be more inclined to choose DL over AA if the schedules and fares work out. And the A220 could play a big role in DL (hopefully) capturing more traffic from DFW in the long run, especially with the number of A220 flights being routed to DFW. Now, if they could only add DFW-SEA in, say, the next year or two...
2019: DAL, MCI, PHX, LAS, DFW, SAT, ORD, SLC, SEA, DTW; B73G (WN x3), B738 (WN, AA, DL), A20N (NK), MD83 (AA), B788 (AA x2), CS1 (DL), 739 (DL), 712 (DL)
Next: AA: DFW-PHL (752), PHL-MIA (763), MIA-LAX (77W), LAX-DFW (789)