Either day won't be as crowded as the Friday or Saturday before Halloween. I think that if you went either night you won't have to worry about crowds or waiting because it is a week night. Also, the 30th and 31st will be cheaper than visiting on a weekend.
There are drawbacks though for not visiting on a weekend. In my opinion, the event is a little more lively when there's more people in the park. I went on a Sunday one year and I was a little disappointed because it wasn't as lively as it was on a Friday. The parade was a little less exciting because there weren't as many people fighting for beads. Bill and Ted's didn't have as much crowd participation. There were even less scareactors in the street. The lines for the haunted houses were long though.
I would say that if you visit on the 31st you may have a little bit more atmosphere since it is the actual Halloween. The crowds for either night won't be too bad since it is a weekday.