I think Valbuena/Stewart will both be allowed to start at least once in a while, and that their relative performance will impact how that goes over time.
If Stewart can hit, and does so better than Valbuena while Valbu continues to hang out in the .220's, then I think Stewart might in time take over as the primary guy. But I think Valbuena will remain the primary guy until/unless Stewart shows that he can contribute offensively. Obviously if Valbuena could boost his average over .260, while continuing to walk and OBP, there'd be less reason to give much look to Stewart.