RE: Pristine vs Major vs. Common Reserves -- in my experience there is little noticeable difference in yield among these three types of system, but avoid Low and Depleted. I have found that of more importance to tons/hour is the density of the ring/ringlet you are mining. Once you drop into the ring, if the rocks are widely separated, you'll take longer to find and mine the rich lodes than if the ring is dense and crowded with rocks. Dense rings/ringlets are more fun as well, as flying requires more attention and spotting the cores is trickier as they "hide" more effectively.
As you approach a ring in SC select one of the more solidly colored ringlets to drop into, they are usually more dense than the others. Rock density can change as you cross over from one ringlet to another (hard to notice "boundaries" when you're in among the rocks) so I drop into ringlets that are both solidly colored and relatively broad, then don't deviate widely from a path that lies along the length of the chosen ringlet. You can use the hotspot center as your "guide star" to keep close to a chosen path.