I've participated in this discussion before and always appreciated Darkbeers extensive time and research on this topic. I can see it both ways and it's a good excuse to celebrate for two days instead of one.
But that being said, for sentimental reasons July 18 will always be opening day to me because that is when our family went. And it coincides with my brother's (Lost Boy) birthday. We were not able to be part of the specially invited guests but we are part of the "general public", the people that Walt built the Park for.