Software-as-a-service (SaaS) is a very successful model for providing cloud-based services over the internet. However, due to the dynamic nature of SaaS services, it becomes very challenging to ensure provision of scalability, applying frequent maintenance and functionality updates to SaaS Services. SOAP and REST are the two mostly used software architectural styles for accessing and consuming SaaS services in cloud environment and each have its distinct advantages. Therefore, to address above mentioned challenges, it is critical to choose the suitable architectural style because the success of a SaaS is strongly coupled with its architecture style. Choosing the right software architecture for a system is a multi-criteria decision making problem and it takes into consideration the architectural style characteristics, non-functional requirements and working domain requirements. In this paper, we propose a rule-based multi-criteria decision support system (DSS) for a SaaS application architecture selection. Our proposed DSS uses weighted sum model (WSM) that take into account the architectural style characteristics, non-functional and domain specific requirements.