I skipped the parties for a few years, opting for something else special like dining at Chef Mickey's and what not. Then I started working there, got to see the parade preview, and of course a small discount on the tickets. After that first time, I was like "why didn't we do this before now, this is awesome."
It really is a personal preference, and you just have to experience both to find out what you like more. Both options are fun, but my preference is the Party. I've gone twice to a party on Nov 2, and while the crowds were still high, I didn't find it to be chaotic like it is the week leading up to Halloween night. Of course your average day guest attendance is about 45,000-55,000 during the day, and the max sell out for a party is 32,000. Overall max capacity is some where in the 80,000 range. Which is July 4th, Christmas, and New Years Eve.
I would also save a bit of money by cutting park days down a day or two. Since you can fill a day at the water parks. But it also depends if you want to go commando style in the parks, from open to close. Or do half park/half water park. Just remember, you can use two or more WPM entries in one day. So you could literally spend from 10am-4pm at Blizzard Beach, go have a round at WinterSummer Land. And cap the night off at Disney Quest.
Disney does heat the water to a lovely 80 degree temp, or somewhere around there. It's just getting out and having the cold air hit ya, that is not so much fun. Just keep an eye on the weather leading up to your trip. If it doesn't work out for swimming, you can just go to the party instead. I'd add the option to your ticket at check-in so you're not bound to it.