ICRF heating of a toroidal plasma is performed by using slow waves generated by m = +1 circularly polarized RF fields and m = ±1 linearly polarized RF fields. From a direct comparison of these two field modes with respect to wave excitation, ion heating, and effects on plasma confinement, it turns out that circularly polarized fields are far more effective than linearly polarized ones in both achieving higher heating efficiency and reducing plasma loss. Ion heating by linearly polarized fields is accompanied by enhanced cross-field plasma loss, which is, in terms of two-fluid theory, quantitatively explained by the fact that ion convection is driven by a ponderomotive force of poloidally asymmetric RF fields.