Just one person’s opinion... but having done the Plaza Garden party on a
MNSSHP night last year, I likely wouldn’t do it again (and I’m generally a fan of these types of things). Fundamentally, in my personal view of MK crowds, it’s not so crazy crowded that having the reserved area makes a major overall difference in your experience.
You can invest about the same time you’d spend at the dessert party grabbing a spot in the Plaza Garden West (the garden opposite of the dessert party) and have a pretty similar experience - roughly speaking. I like to try to get in that area because people tend to think it’s reserved (it’s not) and the bathrooms next to Casey’s are close by (kiddos always have to go 5 minutes before show time).
Ultimately I’m not totally anti-dessert party during MNSSHP.... but if I had to choose between that and something like the FEA/Epcot party (which is great, by the way, in my opinion), I’d drop the MNSSHP dessert party.