I would do the Halloween party, mainly because I haven't done it yet. I really hope that I get to see the Headless Horseman gallop down the paths of the MK some day before I leave this planet.
We've done Pirates and Pals and it was a lot of fun! In fact, it might end up being cheaper than the party for your group. You get some snacks, pictures with Mr. Smee and Cap'n Hook, you get to see the fireworks and the water pageant, which is a lot of fun too and then there's a surprise guest when you arrive back to the dock. So much fun!
There's a way you could do both. Normally (and this won't officially be known until the first party to see how Disney handles it) you can get into the MK with your party ticket at 4 PM. You could not get a ticket for that day, spend some time at DTD or at your resort, sleeping in and swimming, and then head out to the MK mid afternoon. The cost of the party is cheaper than a full day's ticket, so it is something to consider and see how it impacts your budget. Good luck!