I don't think ISU goes far without both. While the minutes can fluctuate based on matchups and how each is playing, but when both guys can play 20ish minutes, they are both very effective. I don't think it would be possible to do what is asked defensively of either guy playing 30 minutes. They have to expend a ton of energy. Where Ward really can shine is against the long athletic teams that hammer the offensive glass like VCU. Tre and Rob are both physical guys that will clear space, but against those super rangy guys that can get up off the floor they give up some tap outs on the offensive glass.
I wouldn't say our rebounding is necessarily worse with Rob vs. Ward, but it's a bit different. Rob's a lot more going to clear space and the guards have to rally and clean up the glass. Ward's going to typically go up and get the board. Lipsey, Gilbert, C. Jones and Pavs do a good job chasing down boards in space cleared out by Rob and Tre. That was one of the big adjustments after the tournament. Guards started cleaning up the defensive glass, Pavs getting some run, and now with Ward back this is a good rotation of complimentary players.