Winter, for sure!! When you're cold, you can always warm yourself up, but when you're hot how on earth shall you get cooler (without air condition, and I don't have one as most private households over here don't have any) ...
Altough I was born in summer I hate it; I hate the heat, sweating, feeling sticky and being unable to sleep when it's still hot at night. I also prefer winter clothes, and the fact that make-up really stays put is another advantage.