My flying is now about 55% DL/VS, 20% AA (BA silver), 20% WN, 5% other. This is after being EXP for several years starting once the AA/US merger was announced. My experience is as follows:

1. DL has far more C+ than AA has MCE.
2. The DL bulkhead aisle generally have more leg room than the AA ones.
3. When I was DL plat, my upgrade rate was in the 60s as long as I avoided ATL. As a Gold, it is in the teens.
4. Many DL 319/321 have live TV in the seat back without a nasty underseat box
5. DL F on 319/320/321/737 is not as nice as AA 737/MD80, not nearly as much space.
6. VS 333 PE is all I need for East Coast to Europe.
7. For all the whining about skypesos, I get 1.5 cents/mile on DL redeeming for C+ domestically during very high periods. Usually 18k one-way instead of $270-$300. But, I get far better value redeeming BA miles when AA F is available (I am sitting in the PHX A10 lounge waiting for a 2 day advance 20k redemption for a $700 F ticket when Florence messed up my travel to the Southeast tonight on DL).
8. FL >>>> SkyClub > AC
9. I still plan on using my AA miles to Asia, as DL has partners I'm not really interested in flying.