A superconducting magnet system for a 28 GHz ECR ion source (DECRIS) was studied in order to select its parameters and optimize performance. Parametric magnetic models were performed for two design configurations, conventional ("sextupole-in-solenoid") and reversed ("solenoid-in-sextupole"). For both configurations, the magnetic effect of the booster and the steel poles on the magnet performance was investigated from the point of view of critical parameters of the system - currents, fields, and forces. Results of the parametric computations were used to optimize the geometry and sizes of the magnet as well as the magnetic shield, the booster, and the poles. A comparison of the obtained parameters was used to select the candidate magnet configuration for further design and manufacture.