Pick one, just one.
It all starts with a well-defined goal. Hopefully, your goal is not merely to get “a sale”, because a singular sale seems to be a meager goal. It would be better to call that an objective for each ad impression, but wholly inadequate for a meaningful goal. I’m sure that you are hoping, and wishing for more than a singular sale.
To be clear, the primary objective of a single ad impression is to get a sale, however, the goal of a campaign should be more meaningful, time-bound, and specific (i.e. $90,000 in 30 days, 30% increase in sales in 90 days, $20,000+ in profits by May 31st., etc.).
By setting a meaningful goal, that targets a specific bankable value, you will instantly remove the question you have about which metric to base your optimization efforts on. At some point you will realize that metrics like CTR are diagnostic in nature, they inform you of conditions, which is useful, but not your primary campaign objective.
I realize that you are probably referring to the campaign setting that allows you to choose automatic optimization of ads within the ad group based on conversions, or clicks. It really depends upon your goal and the strategy you have chosen to reach your goal. If you have not yet set a meaningful, time-bound, specific and measurable goal, it is doubtful that you have selected a strategy for that goal.
It starts with a goal. Every decision is driven by and measured against that goal. Without a destination, how can you know what course to chart? Start with a goal. And no, to “get a sale” is not a goal, at least not a meaningful one.
So… tell me your goal, if it is a good one then I will answer your question about your optimization strategy. I suspect, that once you have set a meaningful goal selecting an optimization strategy will be easy.
Since your question seems to imply that you are concerned with both achieving a sale, as well as controlling costs, perhaps you should set a goal based on maximizing profits? That way CTR and conversions become diagnostic metrics rather than primary objectives. There can be only one “primary objective” per campaign. So pick one.
