I would see the parade and skip the party. BUT maybe that's just because that's what we did! It was our first trip with DS so we had no idea how he would react- would he nap, would he be wired, ... ? The cost of a party ticket was not worth the what if for us- but it was just us 3 and we will be back again in a few years, probably at Christmas time again.
Or may I present option d: let the 3.5 year old crash at the park. My 3yo will not sleep in strollers, but I brought my wrap and he passed out at the parks by 8 pm the two nights that we stayed "late". It was really, really nice to grab a beer with DH and watch illuminations with a sleeping, snuggly preschooler! Of course, we hadn't paid for a special party ticket for him, so we were free to go back to the hotel at any point having got our fill of the parks and there was zero pressure.
I don't know how quickly the party tickets sell out, but is there any way you could wait until there and see how it's going at the parks before purchasing? If the 3.5 is napping and staying up late, you could always buy a ticket then! Just a thought.