Grass biomass would not change (or would change very little) in the bottom-up model but would increase in the top-down model.
Work Step by Step
In the bottom-up model, change must flow from the grass up the food chain to the bobcat. If this type of organization is in effect, the additional bobcats will not strongly affect organisms down the chain. In the top-down model, change must flow from the bobcat down the food chain to the grass. In this case, a change on bobcat levels ripples down significantly and reduces herbivore predation on the grass, causing its overall biomass to increase.