Well I can't decide which I like better, which is why I do both. I think the climbs on Holstein are steeper, and it's generally much cooler than Napa, if not downright chilly. Napa is frequently very hot, and that final 10 or so miles on Silverado into a hot headwind is usually not fun. However, the post-ride meal (and beer!) and music at Napa is more fun than Holstein.
SK makes a good point - you can come do "The Twist" ride in nicer weather, with nicer company, instead of Napa.