I agree that there really isn't a very clear answer to whether Sunday, Monday, Thursday, or Friday would be better.
From what I can tell, the operating hours looks to be almost the same for Sun, Mon, and Thu (8 to midnight in DL and 8 to 10 (or 9) in DCA). So that's probably not going to be a deciding factor.
Fantasmic: At this point, it looks like it's being presented on Sunday and Monday when you'll be there. It won't be presented on Friday (because of MHP). It was briefly showing on the schedule for the weekdays, but it's been removed, so the only days for "sure" are Sun and Mon (based on when dining packages are available). So if seeing F! is important, you'll need to go with Sun or Mon.
Fireworks: Typically, DL only presents fireworks on the weekends and at MHP. So there will be fireworks on Tue and Fri (during MHP, but you can see them from outside park, but you won't hear the music or see the in-park elements). Outside of that, Sunday is probably the only other day when fireworks will be presented while you're there.
MHP: There are MHPs on Tue and Fri. The parties are a lot of fun, but they are an extra cost. DL will close at 6:00 pm on Tue and at 7:00 pm on Fri. There will be a noticeable increase in crowds in DCA after the party starts.
Crowds: It's hard to say what crowds will be like because HalloweenTime is very popular. My guess (and it's just a guess) is that Thursday will probably have the lowest crowds. Sunday will be crowded because it's a HalloweenTime weekend day, Monday seems to have gotten busier of late, and Fridays are leading into the weekend (morning might not be too bad).