Yep-- we have gone when they were doing the wristbands (and when they weren't and would give you a card to get back in if you needed to leave the area). Don't worry- I'm sure they will have different colors which will show whether you are for the early or late show. You don't just wander around in the area- they escort you to a table to which they specifically assign your party. I know at one point they were only doing one dessert party a night even though they had 2 shows, but I seem to remember a time where they were cleaning out the area and doing 2 dessert parties a night.
One of the times we went we got put on the back row even though the front row had lots of empty tables at that point, and we nicely asked if we could have one of the empty front tables instead and were told no because everything was pre-assigned. Shortly thereafter they wound up putting some extremely tall people at the table in front of us, and I was irritated. Most areas have high tables with short tables in front of them, but just a couple of spots had 2 rows of high (which is where we got stuck that time). Had the people in front of us not all been well over 6 feet and built like linebackers it wouldn't have been an issue. The next time we went, nothing was pre-assigned- and what table you got was completely based on your spot in line. No telling how they are going to do it now. Just have to wait and see the reports from those who attend once it starts. Really most of the tables in there are fine unless you get unlucky like we did that one time, and even that time we were able to scoot DD's chair where she was fine, DH was tall enough it was okay, and I just had a couple of big heads in my pictures and slightly obstructed view.
Seems like last time we did it or maybe the time before, they were only doing one line and they came by with an ipad (or something like it) and were just checking everyone in in line. The old fantasmic dessert party, you could go up to the podium early, and they would assign you seats and then tell you to come back later when the area opened, so once you had your seats assigned you did not have to wait in line again. They just escorted people when they got there, no real reason to wait in line ahead of the area opening and people just kind of staggered in. I could see them doing something like that to try to ease congestion in the area between parties, but who knows.
I wouldn't call myself an expert and mostly they are probably the same. I think the pro and con of the late would be:
Pro: More park time at CA if you do do the later
Con: The area might be a lot more congested when you are trying to line up for the later, since there will be a mass of people leaving WOC after the first show, but also a mass of people lining up for the later WOC.
Can't remember for certain, but I feel like we might have done the later showing once and it was fine. I seem to remember them cleaning everything up as we were in line to get in.