After spending many hours hitting various roid sites/rings, I've found that Pristine/Metallic has the highest conenctration of Palladium/Platinum and Gold. Metal Rich is still very good, but Pristine is the key.
I've also found that rings are better than asteroid fields from an orientation/navigational standpoint. In other words its easier for me to mine in a specific azimuth to avoid re-mining the same roids.