This is dependent upon your niche, but I have seen much better results from boosting posts rather than ads. If your niche is the type that supports natural, organic-looking posts (like a link to an outside landing page/article with some text) and users find it shareable/helpful, then you can get a very good cost per engagement by boosting posts. I was able to reduce the CPE from $0.28 to 0.09 for one of my clients using post boosting.
The primary reason for this is that posts can be shared by other Facebook users, therefore giving you a much higher spread for free.