That is a tough call. We did that last year not knowing it was free. There is more ambiance at night which ads to the atmosphere but if I could have saved $400 on party tickets this year I would have but we are leaving before it is free. I would decide soon though. it is likely that party will sell out.
I'm assuming you are talking just about the parade. There are other benefits to the party like a limited number of people in the park. I bet that Saturday will be crowded.
There are characters you can only meet at the party (Jack Skellington, 7 Dwarves, Princes, Scrooge McDuck, etc.) and snow on Main Street. None of those things will be available without a party ticket.