That's a really hard choice, Nikki!
Have they said, for sure, that they're doing the dessert party this year? The only thing I've seen for sure is that they're doing the Party for the Senses (and with tickets not on sale until late August, I don't expect to see anything official until just a week or two, at most, before then).
My sister and I went the two years ago, and we went last year with our other two sisters. (I think they've only done it the two years). They do have a wide range of desserts, and quite a few different companies pouring different kinds of drinks. And then a fun band going most of the time.
It is quite a bit shorter, time wise, than the Halloween party, but I haven't felt 'rushed' at the dessert event, so it's an ok amount of time for the event that it is.
If I was choosing between the two, I'd go for the dessert party - but my sister and I (just the two of us this year) have already decided we're going to skip it this year -- we could go on two different days - but the second one is the day before the half marathon that we're doing - and we know better (even though we'd have a full 24 hours for digestion/etc), and the first is the day before MouseAdventure, and although we've done it the night before MA both other times, we decided to go for the longer nights sleep this year. Plus, we had a lot of fun because it was all 4 of us last year, and a group of 2 generally isn't as much fun as a party of 2!