Capture the Flag (CTF) challenges are typically used for hosting competitions related to cybersecurity. Like any other event, CTF competitions vary in terms of context, topics and purpose and integrate various features and characteristics. This article presents the results of a comparative evaluation between 4 popular open source CTF platforms, regarding their use for learning purposes. We conducted this evaluation as part of the user-centered design process by demonstrating the platforms to the potential participants, in order to collect descriptive insights regarding the features of each platform. The results of this evaluation demonstrated that participants approved the high importance of the selected features and their significance for enhancing the learning process. This study may be useful for organizers of learning events to select the right platform, as well as for future researchers to upgrade and to extend any particular platform according to their needs.