I always attend the final weekend. As Brian said, a few years ago, Halloween was typically dead. Back in 2010 it was a madhouse. It's really a crapshoot. Personally, between it being Halloween and it being a Friday night, I'd expect medium sized crowds.
If you're only going that one night, I'd certainly play it safe and get an express pass. That's me personally. As the event gets closer, you'll be able to get a better idea of how the event is handling crowds and if express is necessary. You'll also surely have our guide that will be published around the start of the event.