The price for HHN is better for Thursday and the crowds will be about the same as they typically sell out each night. California traffic is always a nightmare. Either leave before 6am or after 9am, and after 3pm to about 7pm or prepare to be in traffic. If there is an accident though you could hit traffic at 11am though so no real predictor. Add 30 minutes to whatever you think it will be. That stretch through downtown L.A. from the 10/60 freeway area to Universal always has traffic no matter when I pass through. Sometimes we take the back way through Burbank to avoid it. Again where you start makes all the difference as traffic goes toward L.A. (west) in the morning and toward the Inland Empire (east) in the afternoon. Travel opposite these and you will be better off.
No matter which park you visit it will be busier on a Saturday then a weekday. Sundays can be either depending on when you visit-August is the ending of the high season and October because of HHN will be busy as well for people doing day/night tickets. HHN gets insane crowds so go at least two hours early (plus travel time) to park, an hour if you are just getting dropped off. You can always eat or get a drink while you wait for them to open.