Argumentation is a reasoning mechanism attractive due to its dialectical and non monotonic nature, and its properties of computational tractability. In dynamic domains where the agents deal with incomplete and contradictory information, an argument comparison criterion can be used to determine the accepted information. Argumentation systems with a single argument comparison criterion have been widely investigated; in some of these approaches, the comparison criterion is xed while in others a criterion can be selected and replaced in a modular way. We present an argumentative server providing client agents with recommendations and giving the possibility of specifying which of the available argument comparison criteria will be used to answer a query; for that, we formalize a special type of contextual query which by the use of conditions allows the server to dynamically change the criterion providing a declarative way of representing users preferences.