That's difficult to say because at the Villains Soiree you only see Villains. We saw the Queen of Hearts, Hook, Maleficent, Evil Queen, Dr Facilier, Cruella, Lady Tremaine and the stepsisters so for any other characters you'll still have to spend time in lines.
If you do the first soiree it also reduces the time you'll have for other party activities. If you attend the second soiree you can maximize party time, but with 2 kids and a baby an event that starts at 11:45 PM might be difficult.
On the other hand, with the Soiree, you'll spend minimal time waiting for the parade and HalloWishes fireworks. The viewing area, especially for the parade, is excellent. Some felt the viewing area for the fireworks was too close to the castle, but we liked it and there was plenty of room to sit down.
Going to 2 parties you could focus on the parade and fireworks at one and meeting characters at the other, although there won't be many villains available because they were exclusive to the soiree last year. You can fit the Castle show, dance parties and trick or treating in between.
As far as choosing between a second party or the soiree that's difficult. I think it depends on which is more important to you - meeting lots of villains and having a good spot for the parade and fireworks viewing without having a long wait, but it will be expensive. Tickets to 2 parties will be expensive, although less that the soiree, but you'll spend more time waiting for the parade and fireworks and there will be fewer villains to meet.
MVMCP did not have an added premium event, just the Halloween parties.