In a service-oriented system (SoS) service requests define tasks to execute and quality of service (QoS) criteria to optimize. A service request is submitted to an automated service selector in the SoS, which allocates tasks to those service that, together, can "best" satisfy the given QoS criteria. When the selector cannot optimize simultaneously the given QoS criteria, users need to specify priorities over the said criteria. Accounting for users' QoS priorities is therefore necessary during service selection. Once specified by the requester, quality properties will be used by the selector to lead autonomic optimization of the service selection process. We outline and test a selection approach that accommodates priorities and that is based on available Multi-Criteria Decision Making techniques.