It is a long-standing debate, whether software that is developed as open source is generally of higher quality than proprietary software. Although the open source community has grown immensely during the last decade, there is still no clear answer. Service-oriented software and middleware tends to rely on highly complex and interrelated standards and frameworks. Thus, it is questionable if small and loosely coupled teams, as typical in open source software development, can compete with major vendors. Here, we focus on a central part of service-oriented software systems, i.e., process engines for service orchestration, and compare open source and proprietary solutions. We use the Web Services Business Process Execution Language (BPEL) and compare standard conformance and its impact on language expressiveness in terms of workflow pattern support of eight engines. The results show that, although the top open source engines are on par with their proprietary counterparts, in general proprietary engines perform better