Every shower head is a minor restriction on flow compared to the bath faucet. It may or not be negligible. But a bath drawn for the same faucet runtime will use more water than the same runtime for a shower every time. The crossover in cost happens when the shower uses more water than the bath. That should happen very shortly after the time it takes to fill a bath. At one house, our bath took 40 minutes to fill, another 15-20, and another took under 10.