I'd say Halloween too because it is so much fun to dress up and even more fun to see the other people's costumes.
Also, if you plan it right you can overlap the Food and Wine Festival if you want it to...
But really, the main reason to choose Halloween for me is that I live in the woods and we get NO trick or treaters. Now that the kids are grown, the only Halloween fun we have is watching scarey movies. Christmas is a big family extravaganza, but Halloween is usually really quiet.
The weather is a bit more reliable in October, as December can be really cold or really warm or anywhere inbetween.
But face it, there is no losing answer here, both are great fun.