Thank you, rufusjack. If I had multiple screens, I may do both, but with a single screen, I'll probably stick with one format at a time. Among other factors, I think that switching back and forth would confuse people. But mostly, I'm betting on a certain "Wow" factor that comes with offering 3D for such a low price.
Comparing attendance between 2D and 3D, though, is tricky because it's not apples to apples. The upcharge changes everything, and it becomes (I think) a huge factor with family films. I certainly believe that some people prefer 2D to 3D, but I absolutely know that people prefer paying $3 less per ticket.
Also, as with any biz, there are sooo many factors at play in determining what will or won't work in a cinema. Remember that "Santa Claus Conquers the Martians" package from a year ago? We drew 80 people to a matinee of that thing (very good crowd for us). We charged dirt-cheap admission and advertised it as "the most Ho-Ho-Horrible holiday movie ever made" - and not as some nostalgic romp. I don't think that would have worked just anywhere, but this area is very receptive to that sort of thing.
So, to me, 3D is similar. There's a market for it, but it probably really helps a theatre if the conditions are right where you're doing business.