Hi, Jodi!
Thanks for checking in with the Disney Parks Moms Panel!
I think you need to chat with your kids to find the correct answer to your question.
Mickey's Not So Scary Halloween Party is a super fun way to experience Halloween with Mickey and many of his friends along with other well-loved Disney characters. There is also a special Halloween-themed parade and dance parties. If your kids want to have special opportunities to meet and greet with characters and ride the rides with a smaller than normal crowd, I'd recommend the party. My kids attended the party when they were 6-years-old and ten-years-old. They loved trick-or-treating and dressing up in Magic Kingdom Park.
I asked my now ten-year-old and 14-year-old which they would pick and both said they would choose the party. But they both agreed the water parks are really fantastic. I know if I had the choice, I'd pick the water park.
I realize that doesn't give you a definitive answer, but I hope it helps you when making your decision!
Let me know if I can help answer any additional questions! I'm happy to help.
-Jen