"Focus on Conversions" vs. CPA Bidding Strategy
In the campaign settings you can choose "focus on conversions" and set a maximum CPA or a target CPA.
In the bidding strategies feature there's an option to optimize based on CPA.
What's the difference? Have you tested both? Which one do you recommend?
Re: "Focus on Conversions" vs. CPA Bidding Strategy
Focus on Conversions = AdWords Conversion Optimizer
With Conversion Optimizer, Google attempts to use historical data to adjust your CPCs, to give you a target CPA that meets your advertising goals. In order to run, Google has to have some historical data of Conversion in order to see how to adjust your Bids. Target CPA = more or less an average of what you'd like to pay per Conversion; sometimes it'll be higher, sometimes lower, but the average should meet your target. When setting a maximum CPA, Google attempts to never have your CPA go above the CPA you've set; often times you'll see your actual CPAs much lower than your maximum, because the system will never attempt to go higher than the amount you set.
Target CPA under Flexible Bidding Strategies has similar goal - give you as many Conversions as possible, while meeting your Target CPA (again, working as kind of an average).
I believe the main difference is Conversion Optimizer you can select Maximum CPA or Target CPA. The Target CPA under the Conversion Optimizer should work similarly to the Target CPA under Flexible Bidding Strategies.
Regarding recommending one, I'd say it's up to you and your business goals. If you know that you can never pay more for a Conversion, you might consider Maximum CPA. If you're looking at scale, and the lifetime value of a customer, you may consider Target. I typically see an overall increase in the number of Conversions when using Target.
Does that help?