I asked
Luke@Tirerack about both of these tires. He said I would be VERY disappointed with the handling performance of the ContiExtremes. He would not recommend those for an SVT; it was my impression from his comments that they have too soft a sidewall for any kind of spirited driving. The PZero Nero M+S, according to Luke, was a very good performer for an all-season tire; perhaps the best for the twisties. Though he also stated it would probably not be the tire you would want to drive regularly in snow.
Luke offered another option, the Yokohama Avid H4S. It was cheaper than the Pirelli, had a better treadwear rating, would be better in snow, and offers much more handling performance than the ContiExtremes. The Avid H4S is the tire I purchased about 15k ago so I am slightly biased. They have been great tires but I cannot as of yet speak to their winter performance.
Call Luke at TireRack, ext 362; he's always very helpful and might even offer you an option you hadn't been considering.
