Hi Todd,
How fun that you and your sister will be taking a trip to Walt Disney World together! If I were going with one of my siblings, I'd attend
Mickey's Not So Scary Halloween Party in a heartbeat! The experience is not something you can or will get from a regular visit to the parks, it's a unique evening of fun and excitement in the Magic Kingdom.
By attending this special event, you will be treated to unique decorations, spectacular displays and street entertainment as well as the incredible "Boo-to-You" parade and "HalloWishes" fireworks which are worth the cost of admission alone. Of course you'll leave with a bag full of goodies if you wish to trick-or-treat during your visit.
Personally, my kids are a bit young to truly appreciate this event (so being thrifty, I'm waiting until they are older to take them again) - but I would jump at the chance to visit with another Disney-loving adult! There is something special about these events, I don't know if it's the detailed decor, the lower crowds (compared to a "normal" day), the spectacular entertainment or the attractions!
Hope this helps - good luck with your decision!