Have done both of the parties and have really enjoyed them. Was listening to a podcast yesterday that was talking about them and one tip they said was to try and book Sunday night if you can, as these are usually the least crowded. If you can't do Sunday, try Monday through Wednesday. They said Thursday through Saturday are normally the most crowded, especially the weekends as that is when locals go.
If you go to the Halloween party, be prepared to receive a ton of candy, if you "Trick or Treat." My wife and I did this and ended up taking it to work and giving it away because we received so much. They just dump it in your bag in many cases.
I prefer the Christmas party as I think this is the best time to be in the parks. There is just something different at being at Disney at Christmas time. I do think you can get most of the feelings of the party without being at the party, just in the park on a random day. But the party is a lot less crowded in a time when the Magic Kingdom tends to be crowded.
I too think they have upped the number of tickets sold to these events in recent years, but you can definitely tell its not a normal park crowd, especially if you go on a "down" crowd night.
If you go for a Christmas party, also make sure and take a trip to Hollywood Studios and see the Osborne Spectacle of Lights. Its a pretty amazing display (several million lights, lights dancing to the music playing) and this is the last year they will do the display.