I'll echo Robin's comments.
Andrew and I ran the Silverstones in 2002. They were ok, but we were forced to run very high tire pressures to compensate for the weak sidewalls, and we still had more flats that year than I think is reasonable. I think Silverstones may be appropriate if you're just out there having fun. If you're trying to win, they appear to me to be false economy.
As between the Michelins and the Pirellis, I'd give a slight edge to the Michelins. In my experience, in the same compound, Pirellis tend to wear less than a Michelin, but once a Pirelli is half-worn it seems to lose a lot of grip and be basically worn out. Michelins seem to stay sticky until there's basically nothing left. And clearly, in the US, the price favors Michelin.
- Christian