Today, companies selecting an ERP must decide between SaaS- and On-Premise-ERP. This special topic of deciding between ERP operation modes has not been addressed since the first SaaS-ERP system was introduced. To provide guidance in the selection of ERP operation modes, systemic differences between SaaS and On-Premise have been researched and considered together with the company’s characteristics. For this purpose a short list of all main systemic operation mode differences is presented. These differences are the starting point for matching the company characteristics with the respective Operation mode. 7 single case studies of ERP customers have been conducted and analyzed for the purpose of developing these claims of superior fit and offering first evidence of their validity. The case study results, as well as the developed claims, show the ERP selecting reader which characteristics fit best with which operation mode, enabling him to take all main criteria into consideration.