I had been to Ventana Big Sur, going to Alila Napa next month. Personally, go for Big Sur if you are not really into wine tastings and just want to relax and getaway from everything. Big Sur is really a place to disconnect from everything, you can experience the nature (hiking, the views, picnic in the forest). I think this is a place that's perfect for a couple's getaway.
Meanwhile in Napa, you can go do wine tastings (a lot of top tier wineries nearby), go to Downtown Napa or Yountville (this is a foodie's heaven, a lot of great restaurants nearby)
Personally I would spend more time in the hotel room in Big Sur, I would spend more time outside of hotel in Napa due to wineries and nearby restaurants.
Can't really go wrong with either one, it all depends what you are looking for. Although I think Big Sur's availability is harder to come by