If you let the water drain to one floor level below, then you'd need to spend energy pumping it back up, plus the height of the toilet. Installing a tank above the toilet, and pumping to it from the tub, would be "easier". That is, unless you're filling a 100 gallon tub daily. In that case, using an outdoor reservoir would be more suitable, and easier than pumping it up ~15' after draining to a basement. If you're looking at the "ease vs difficulty" of a one-time install, then you're not paying attention to the long term factors.