Knotts by far has the nations biggest theme park Halloween nights. They've been doing it for decades, longer than any other. This is the park others try to follow for Halloween as they started it all! That also means its the most crowded on many of those nights. Many of the rides are themed with effects for the night. They have 13 haunted houses, several mazes and scare zone themed areas to walk through. It's not for young kids. Older teens and adults only. Arrive early or risk getting stuck in traffic around Knotts. Its an awesome event and extremely popular!
Universal puts on good Halloween nights as well but is much newer at it. Not nearly as crowded as Knotts. They don't have haunted houses here but they do have scare zones that are better themed than Knotts. They also have several haunted mazes and a haunted tram tour.
Hard to say which park is better as they're vastly different experiences. I assume you don't have small kids as the neither is particularly good for them but Knotts would be the better choice during the day as they do have a decent kids ride area. I'd say older teens and young adults would prefer Halloween nights at Knotts while slightly older adults would prefer Universal. Both are excellent though!
