I have to say Christmas because I get joy out of seeing my family all come together, it's also one of the only times I get to see my sister due to her living in another country :)
True, I spend lots if time with my family on Christmas, which is probably one of the main points or Christmas. I also like the decorations that go up. Also another time that you can spend time with your family. I usually like decorating the tree.
Halloween is incredible in terms of atmosphere but no holiday is at the same level as the corporate moneymaker dubbed Christmas. I love meeting friends and family, singing, exchanging presents. It brings everyone together.
Halloween, while having truly respectable origins, is just a "holiday" for kids to dress up and gorge themselves on candy. While it is important for children to express themselves, especially through attire, I would argue that limiting that expression to one day out of the year really inhibits the creativity of a young child's mind. For that reason, I'd have to go with Christmas.
Late to reply, but no doubt, Christmas. Halloween stops being fun past 20. Christmas? Always fun, always getting gifts, always giving gifts, family, food, etc.