I don't know that either day is going to be markedly different, other than as you mentioned, Friday is the first day of HalloweenTime and Dapper Day.
Friday: Based on the Disneyland.com calendar, Friday actually has longer hours than Saturday (one hour). Full evening entertainment is running (F!, fireworks, Mad T, WoC). None of the APs are blocked, but it a work/school day. I would expect moderate crowds in the morning/afternoon and then an increase in the evening as locals come to the park.
Saturday: Full evening entertainment is running. The SoCal and SoCal Select APs are blocked, but the Deluxe, Premium, and Premier APs are still valid. It is not a work/school day, so guests can start arriving in the morning. I would expect moderate crowds in the morning (first few hours) and then an increase in crowds throughout the day as locals come to the park.
If you're already going to be there on Wed and Thu, I would probably wait to see how you're feeling on Friday. If you ready for a sleep in/relax around the hotel=type of day, then I would skip Friday and go on Saturday. If you're still feeling energetic, then I would go on Friday. (Personally, I would probably upgrade to four-day tickets and go on all days!)