Well, after going back and forth with myself, I decided to get the 15-disc set, as there was enough there that I didn't have yet and wanted. I can't really blame AB and SF for using so much of what had already been released, as a lot of it is very good.
However, there are a few oddities. There is a definite audio sync issue from about 32:00 - 56:00 on Halloween 4; it seems to be a drift, as the sync goes further and further out as it goes. Then it sort of drifts back in sync after that. On other forums, it's been brought up that there was a slight sync issue on the previous AB Blu-ray, but that it wasn't this bad. I don't have that one, so I can't compare, but I can say that on both commentary tracks on the new disc, some of the dialogue during those scenes is audible, and it is perfectly in sync. So something obviously happened here.
Also, the 2.0 track on Halloween II (the original "II") is dual mono, rather than stereo. Apparently, it was this way on Shout's original disc, too.
And the extended TV cut of the original on disc 15 is HD, except for the TV scenes, which are SD, but look like they've been upscaled pretty decently. They actually don't look all that bad, considering. However, for some reason, the 5.1 remix has been used for the theatrical scenes, and then the TV scenes sound like they've been expanded slightly out to 5.1. I guess this was deliberate, but it seems like a really odd decision not to just use mono for the whole thing.
Other than that, it's a great set; which makes the nitpicky things more frustrating, since they're so basic.