Systems of Differentiation should employ a build-first strategy. These are the systems that differentiate you from your competitors, they are your competitive edge. As such you typically need tight control over these systems and don't want to rely on 3rd parties - either vendors or contractors.

Systems of Engagement & Systems of Record should employ a buy-first strategy. These are typically more operations-focused systems and as such you're usually better off buying them, but not always (see below). It's possible that some of these systems may also be Systems of Differentiation.

What is a buy-first strategy? It means consider buying first. How much would your operations be impacted by the necessary changes to your workflow? How much customization will be required in order for it to be usable and how are those customizations maintained throughout product upgrades? How much integration is required with existing systems? Buying software is rarely a buy, deploy, and done proposition! Usually the TCO is lower if you buy, but not always - depending on how much customization and integration is required.