The purity vs. quantity decision should be made based on the amount of Salvia leaves you have easy access to, and the amount of Salvinorin you wish to produce. My most recent batch of Salvinorin yielded hundreds of mg of Salvinorin out of only 6 ounces of leaf... this is far more than I could possibly use any time soon. Additionally, I was able to take not-so-very-clean gummy-wax Salvinorin and clean it up very nicely to the point of beautiful crystals. It's not that Salvinorin is lost - it's a matter of where it goes.
The decision is identical to squeezing the juice out of an orange. If you want the maximum juice per orange, you're going to squeeze the snot out of the orange, and get lots of pulp in the process. With enough straining, you'll get the same juice as if you just took the easiest-to-reach juice out of a lot of oranges. The question is how many oranges do you have to spare.
When making the purity vs. quantity decision for the first time, consider going with purity first, and saving all of the "discarded" substances separately for a separate batch. This will get you the most clean Salvinorin in the least time. The separate batch will potentially create salvinorin just as clean as the first, but will require a lot more time and intermediate steps to get it out. Unlike oranges, the "discard" substances will stay pretty much the same, and if you store them in a cool dark place, you can always extract them some time later. If Salvia leaves are in plentiful supply, you may as well opt for straight purity, throw away the discards, and forget the effort of trying to squeeze the last bit out of every leaf.
Steps of the Salvinorin extraction process that favors quality over quantity: