One of the things I like best to do at night is soak in hot water and read a book. Would it be more efficient to have a hot tub that runs all the time (and is outside) or get a bigger tub for inside but have to fill it each night? (I was thinking of adding a tankless water heater at the tub area. I have propane.)
Either one is likely to consume a substantial amount of energy.
A relevant question is how well insulated would the outdoor hot tub be? If you go to extremes here (I knew a guy who had a 12" thick top he lowered into place with a sort of crane) you can substantially reduce the cost of keeping the water at a selected temperature.