here i can give a good answer, since this is basically a physics question.

if you want to turn electricity into heat, your energy conversion efficiency will be close to 100%. no matter what system you use.

in contrast, if you want to use electricity to pump water up a mountain, you will have losses in the form of heat, and not all of the energy will actually end up moving water upwards.

but what losses could there be in this case? it gets warm, but it doesnt produce light or sound or radiation or mechanical energy or anything else.

so if the setup is the same and you heat to the same target temperature, electricity cost should be the same. it could only be different if, for example, one method heats more air surrounding the enclosure, or also heats a wall of your room, and the other method is better at delivering the energy to the underside of the tub.