If your existing gas water heater is near the end of its life you might also consider replacing it with a high output gas water heater. High output units have a much larger burner, a better tank and a longer warranty than a standard water heater. The price difference should be an around an extra $200 over a standard water heater.
If you do get a tankless water heater, I agree it should become the main heater for the whole house to benefit from its extra efficiency.
Several people I know that bought homes with a big tub admit they only use it a few times a year if they use it at all.