I've done a Halloween trip down there the last four or five years and have done Disney, Universal's Halloween Horror Nights, and Knott's Scary Farm.
Universal Halloween Horror Nights - Teens and adults only, it's intense and a long night, if you have teens that don't like horror stuff but "will give it a try", seriously think again, there is no re-entry to this so you can't walk them back to a hotel to drop them off. Speaking of hotels, I HIGHLY recommend staying at the Hilton or Sheraton across the street (in Universal City), especially if you're going to be in the park during the day and then for the night time event. Traffic from Disney can be as bad as 90 minutes and that's without any accidents, traffic home is better at 30 minutes or so, BUT, it's probably going to be at midnight, 1 am, or even 2 am, think about how worn out you'll be and weigh that against the (relative affordable) cost of one of those two hotels. Get the front of the line pass or RIP pass. Front of the line will allow you to get to the front of the line ONCE for every maze and the attractions that are open during the night (not all are). Universal is fantastic about it actually being "front of the line", wait times are usually 5 minutes or less with this pass, even on the busiest nights. The problem with that, is you actually burn through the mazes long before the night is out. Don't get me wrong, it's a huge event and really well done, but each maze only takes 5 minutes or so to get through, and there are usually 8 to 10 of them plus a show and random scare zones around the park. So, if you're getting front of the line, kept the badge (bar code they scan) in your pocket and use the normal line during the start of the night up to whatever your limit is (20 minute wait, 30 min, etc) or until about 9pm, then bust out the front of the line and go experience some mazes for a second time and make sure to hit everything at least once. OR, the expensive option, get RIP. It includes valet parking, a very gourmet buffet dinner (like sushi items, hot meat stations, pasta dishes, etc, really good!) and a guided tour with about 14 people max and one tour guide. They'll make sure you get good seats to the show and hit every maze at least once, usually by 11pm or so. After that your badge allows for unlimited front of the line, so if there is one favorite maze that you love and has a bunch of detail, you could do it ten times in an hour easy before heading home, or go do everything two or three times, etc. Depending on the nights these passes are about $300 to $350/person. Usually they also include "early entry" where select mazes open and during when normal attractions are open, I think at 4pm. The park "closes" at 6pm I think and the Halloween event starts then.
If the price is just too much and you're buying a normal ticket, early in the season (early October, or even September) is usually manageable crowds and AFTER Halloween is usually better. (I suspect this year they'll just run one weekend after Halloween, so Nov 1st - 3rd will probably be the end) Past that as close to midweek as possible, except this year for the week OF Halloween. Good luck, but save up and buy a pass!
(To give you an idea of what's do-able with Universal's "relaxed" VIP treatment.... I'll usually do the park in the day with a VIP pass, starting at about 9am. It includes a nice little finger food type breakfast in the lounge, and a nice gourmet lunch at a private location. The tour usually ends around 3pm and people can break off and do unlimited front of the line for whatever they want. I'll hit a couple things two or three times and then drive across the street to the Hilton around 4:30pm. Usually shower and change into some non-sweaty clothes if it's been hot, or put on another layer if it's going to be cool out. By 5:30pm I'm driving back across the street to valet the car with Universal and pass security [they have LOTS of metal detectors setup by then, probably 20, but lines can still get long as the night goes]. Meet up with my tour group by 6pm when it's just starting to get dusk and request to be put in a group that is heading over "early" for dinner. Then usually my group is heading toward mazes by 7pm and I'm in the park till ~1am. I can do every maze at least twice if not three times in this amount of time. It's an expensive and long day and I sleep REALLY well when I head back to the Hilton, but I've been in the park for ~15 to 16 hours and don't feel like I want to die or that my feet went numb. VIP during the day and RIP during the night makes it really do-able in a non-miserable way, if you want to spend the money and do both things.)
Knott's is "2nd tier" to me, partly because of the crowds. I feel like there are A LOT more 14 to 18 year olds over there, screaming around either honestly scared, or just being stupid with their friends. It's probably because the ticket price is a lot cheaper. The mazes are good, but not as good as Universal. They tend to be "home made" in the sense that it's not based on a TV or movie property but instead just custom made. Most the Universal ones are based on some movie or TV property, except for one or two every now and then. Lines for some mazes can get insanely long at times so a front of the line ticket here is worth it too. The park is less decorated for Halloween and is extremely dark, I was frustrated and mad the first time there because I kept finding myself going in circles since I didn't know the park layout and it was too dark to read signs or the map, and if I put my head down to use my cell phone to light up the map I seriously risked getting jump scared by the roaming scare-actors. After the event I realized I was more on edge than any other time, I actually really enjoyed that, but in the moment trying to find a maze for 45 minutes while somehow ending up at the wrong spot five times in a row was maddening, lol. Knott's is about ~25 minutes or so from Disney on most nights and I probably would suggest staying near Disney during that time. Unless you plan on doing the park in the day, you could do Disney for over half a day, leave at about 4pm and get to Knott's and into the park by 6pm for the Halloween event. Then either drive back to your hotel near Disney, or, use this time as the transfer to a Universal City hotel since traffic will be empty at midnight or 1am.
The worst wait times I've seen at Universal Halloween Horror Nights is 180 to 200 minutes for some star mazes like Stranger Things.
Worst times I've seen at Knott's is about 140 minutes for one or two knock out mazes but ~80 to 90 for the rest.
I usually go the week before Halloween (so ~19th to 26th time frame give or take a day)
When are you planning to be there?