If you already own a Galaxy Nexus there is absolutely no way I would recommend trying to switch. If you are shopping try to choose between the two, there are advantages to either, all worth considering.
The Galaxy Nexus has unmolested ICS, a higher screen resolution, more support if you are into rooting and flashing ROMs, replaceable battery, and great accessory options already available.
The One S offers a better camera, Sense (which is nice and does offer useful features), smaller overall size, and faster data speeds on T-Mobile service.