No way Avatar attacts "Potter" opening day crowds, unless Disney markets the crap out of it, and ALL the Disney Nerds show up. However, considering a lot of the Disney nerds seem "less enthused" about this attraction, I don't even know if they will show up. At least not all on opening day.
There are no Avatar nerds, or if there are, they are a very small minority.
There will probably be more people at Star Wars Weekends then at the Avatar opening...