Data migration is a one time activity done when a customer moves from one HR system to another. For example a customer may move from OnPremise HCM to Employee Central. They may choose to move all their employee data from SAP OnPremise to Employee Central. They will use a standard professional services package provided by SAP to do this.
Integration is an ongoing exchange of data between SAP and SuccessFactors applications. For example integration of employee master data from SAP to SuccessFactors.