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For the past two decades, representative institutions, political parties, and social movements have been using internet platforms to increase citizen participation in decision-making. This use of digital tools in participatory processes—whether consultative or co-decisional—has raised questions among sociologists and political scientists. A recent conceptualization distinguished two categories of ‘digital parties’: essentially centralized and consultative ‘platform parties’, and more decentralized and co-decisional ‘network parties’. How is this distinction reflected in platform design and online debates? Based on a comparative analysis of the Rousseau (5 Star Movement, Italy) and Decidim (Barcelona in Common, Spain) platforms, this article shows that the organizational and ideological differences between the two party types are reflected in the platforms’ architecture, and in the ways in which users debate online.