1. A computer-implemented method comprising: obtaining a plurality of model representations of predictive models, wherein each model representation is associated with a respective user and expresses a respective predictive model; and selecting a model implementation for each of the model representations, each model implementation comprising one or more computer programs operable to be executed on one or more computing devices, each model implementation being selected for a corresponding model representation based on one or more system usage properties associated with the user associated with the corresponding model representation, wherein the one or more system usage properties comprise one or more of the following: (i) an amount of system resources that the user has consumed over a given period of time, (ii) a time of day during which system resources were previously used by the user, or (iii) an amount of money that was previously paid by the user for the system resources.