Typically, hot tub heaters are controlled by a thermostat that turns them on when the temperature drops to the bottom of the desired range and turns them off when the temperature reaches the top of the desired range. The heating element should consume the same amount of energy per day regardless of its wattage: it will spend more time "on" at lower wattage, but watts x time will be very nearly the same in either case.
Thermal mass of the water, and the starting and ending temperatures will of course determine how much heat (heat = energy = watts x time) needs to be added to the water; but low wattage just means slower more time is needed.
If there is a lot of heat loss, though, (e.g., you're trying to keep the tub hot all winter, there's no cover, and the walls and bottom of the tub are in snow) a very low-wattage heater might not be able to keep up with the heat loss: even running constantly, it might not be able to get the temperature up to the desired range.
Edit: Normally "efficiency" would mean the percentage of heat that gets into the water, instead of being lost to the air. In the short run a higher-wattage heater would lose a bit less heat while heating the water, but it will all average out to about the same because in that case the water is at a higher temperature for a bit longer time, and loses heat faster when hotter.-end edit
Probably any hot tub pump will move water fast enough to keep up with any heater you use, but you should check the instructions on the heater to be sure.