The current era of (e-)learning invests heavily on Massive Open Online Courses (MOOCs). Various dedicated MOOC platforms are used, such as Coursera, Udacity, and Open edX. Moodle, one of the most popular open-source Learning Management Systems (LMS), has been also used for MOOC implementations. Usability and user experience (UX) are very important for e-learning software. This paper presents an in-depth comparative UX evaluation of two platforms used for MOOC implementations: Open edX and Moodle. The reported evaluation study combines user interaction metrics and wearable biosensors. Participants performed four typical interaction tasks in both platforms. User performance data (task success, time on task), self-reported data (SUS, AttrakDiff), and stress-related physiological data (skin conductance, heart rate) were collected and analyzed. It was found that Open edX performs better than Moodle in almost every UX aspect we examined. However, the reported results do not imply that building and delivering a MOOC via Moodle is prohibitive. The choice of the platform to be used by an institution is affected by several factors of non-technological nature, such as students' familiarity with an e-learning platform, available financial and human resources, as well as delivery deadlines.