The European Solar Telescope (EST) is a 4-m class solar telescope that will be installed at the Observatorio del Roque de los Muchachos in La Palma (Spain). Two important factors make EST different from other similar size telescopes: high thermal load due to solar radiation and open dome configuration during observation. Therefore, the entire telescope will be exposed to direct and indirect solar radiation, which requires a very demanding thermal management in order to minimize local seeing. In addition, the complete telescope will be exposed to wind, which has an important impact both in image stability and image quality. The EST M1 Assembly (primary mirror and cell) will have to cope with these critical issues, apart from other aspects common to primary mirrors of similar size. In 2020, in the scope of the EST Preliminary Design Phase, the development of the M1 Assembly was granted to the company SENER-Aeroespacial. The first step, before starting the design, was to evaluate different technologies and configurations for the following critical aspects: mirror (lightweight or not), number and type of axial actuators, cooling system, and number of interfaces with the elevation tube. A comparative study was carried out in order to define the baseline configuration for the M1 Assembly. This baseline configuration was then fully developed up to preliminary design level. This paper addresses the comparative study performed for selecting the most promising baseline configuration for the EST M1 Assembly.