With reference to previous comments about the weight of wheels, smaller ones are typically lighter. I know, d'oh, but that is one downside to today's big wheel fashion. For example, my cheapo 17 winter alloys are significantly lighter than the Motegi or Enkei wheels mentioned above, at 1/2 to 1/3 the price. Yes, they are not as nice looking, depending on your taste.
Another downside is the very low sidewall on the low profile tires these large wheels require, which increases the chance of damage from potholes and other road hazards - particularly important in a winter setup.
Flymo