I may be bias..but I would choose
MNSSHP. We just love this event! HOWEVER....here are some thoughts (pros and cons)
Depending upon what time you arrive - you can usually get into park around 4-4:30 with MNSSHP tickets even though it starts a 7 giving you some extra park time so almost like having a 4th park day..
If you are traveling a long distance or from early in the morning you may get tired during MNSSHP and either find yourself leaving early or being cranky especially if you're traveling with kids.
If you stay late for MNSSHP you may be exhausted the next morning (especially having it been a travel day as well) and not get as early a start as you wanted for your first full day or start your day being tired.
If you want to wear costumes to MNSSHP ( you don't have you) you may want to pack them in carry on (if flying) to have them for party just in case your bags aren't delivered to room on time.
The big question here is that you want to go to the party on you a travel day. If you are delayed arriving at Disney will that affect your timing going to MNSSHP? Will you be tired from traveling? Are you traveling with young kids who will be tired? Do you have time to rest when you arrive before going to MNSSHP?
We LOVE MNSSHP - in fact we make a separate trip to see it. I think it is worth going to and since DS loves villains it's a must see for him. But I think you should take into consideration the fact that you will go on a travel day. Weigh the pros and cons and then decide.
Based on what parks you want to go to, PH on a 3 day trip can help you see a bit more..either do 1/2 days in 2 of the parks so you can visit all 4...or PH to favorite park on 3rd day. If you think you may only hop once during your trip and going to MNSSHP on arrival day will be too much then maybe do neither and use the money for a special treat the day you arrive like a nice dinner or some extra shopping in DTD.