Did you already buy your tickets to MHP for 9/25? It's sold out online, but they may still have some if you call (714-781-4400).
This year's MHP ticket is good for entry into both DL and DCA, starting three hours before the party. On Monday, the party starts at 6:00 pm, so you'll be able to get into DL or DCA starting at 3:00 pm.
DLR does
not use Magic Bands, but they do still have FPs (and MaxPass, which is a paid version of FP). Historically, FPs have not been available in DL during the party (so the last time windows end at 6:00 pm in DL). DCA will likely offer FPs throughout the operating hours, through they may run out before the park closes (especially for the most popular attractions). My guess is that RSR and
GotG: MAD will be the most popular. They said they won't start distributing GotG: MAD FPs until 3:00 pm, so there is a good chance you'll be able to get one if you go to DCA first,
but the attraction won't switch over until 7:00 pm, which is after the party has started in DL. All of the party exclusive activities (trick-or-treating, parade, fireworks, Cadaver Dans, etc.) will be happening in DL, so most party goers will probably head there at the start of the party.
In DL, the most popular attractions will likely be Haunted Mansion Holiday and Space Mountain: Ghost Galaxy. If you've never been on either, I recommend trying to ride both during the overlap period (3-6 pm).
Personally, I would focus on the party exclusive activities and forgo the attractions unless you have time.