The Plaza Garden party has a large capacity and you generally have a lot of time to book. Although I didn’t closely follow booking patterns on
MNSSHP party nights last year so I don’t have a good feel for if/when they eventually may sell out. We did the Plaza Garden version around a Labor Day weekend MNSSHP last year and it was pretty darn empty.... like
really empty. Nowhere near a sellout.
The Tomorrowland Terrace party has a much smaller capacity and is very prone to selling out. Even moreso if it really does have parade viewing this year. I’d be on that like a hawk if it’s your choice.