I would
not plan on or count on shorter lines. It COULD happen, but in 2010 I went with the expectation of an uncrowded park and short lines. It was probably about 30 minutes each for Haunted Mansion Holiday and Space Mountain Ghost Galaxy, and for some reason the line for Big Thunder Mountain Railroad was outside the switchbacks!

Granted, without Fastpass you wait longer for Space Mountain for sure, but I don't consider 30 minutes short. If I would have expected large crowds, I would not have been disappointed.
The atmosphere is neat (though again, more crowded than I expected), and if you love villains or Nightmare Before Christmas, the fireworks show is excellent. Plus, you can get into the park at 3 or 4 PM on the party day, so you can get a pretty decent-length day at
Disneyland. Still, if you're not going to dress up, don't want to trick-or-treat, don't care for the parade, and aren't into meet-and-greets, I think you're mostly paying for fireworks and some nifty scenery. Well, and some great atmosphere of seeing other folks in great costumes! (If you have a park day when you're not going to the party but are in Disneyland until the party starts, you can see quite a few of the costumes from 3-6 or 4-7, though.)
I've not been to Universal for years, and certainly not during Halloween, so I can't help you there. If you take a day trip to Universal, I'm not sure how the haunt setups affect the day guest's experience.
If you are truly undecided and wouldn't be completely disappointed to miss the party, you could see if tickets are available closer to or even on the party date and decide then.
Have a great trip, wherever you go!