My hot tub is a convertible 120V or 240V, in 120V 20 amp mode the heater runs at 1,500 watts and it only runs the heater or the jet pump, but not both at once, in 240V mode the heater runs at 6,000 watts and will run at the same time as the jet pump. Of course it is a small 3 person tub (really 2 if they are friendly), and I have it set up in 120V as I already had a 120V 20 amp outlet available. When I bought it I thought I would have to wire it up for 240V to be happy with it, but the truth is 120V works fine for me, it just means I have to always keep it hot and plan ahead for water changes as it takes nearly 24 hours to get back up to temperature from a cold water fill. Heat loss while the jets are running is a non issue for me, as the tub also has heat recovery feature from the running jet pump so it stays fairly stable, and does not drop more than 1 -2 degrees after 30 minutes with the jets running, which is about as long as I ever run them, even in cool weather.
If at all possible I would pick out the hot tub before doing the electrical, there are just too many variations on what you need ran to the tub, the common ones are:
120-15 amp (mostly small tubs, this is your common household outlet)
120-20 amp (the is the one you find has the T shaped slot on one side that you can plug 15 amp or 20 amp cords into, most people don't even know what a 20 amp plug looks like though they are that rare) NEC code now requires 20 amp outlets in most businesses, although you will find some in homes.
240V-50 amp, most common traditional hot tub requirement.
240V-60 amp, some newer hot tubs, mostly larger models
120/240V combination, such as mine if I were to convert it to 240V, it would require 50 amps of 240V service, however that would be split into a 240V 30 amp breaker, and a 120V 20 amp breaker. This is common on all 120v / 240v convertible tubs made by Hot Spring since they use a 120V only pump motor, but a convertible dual wattage heater.
There are probably others out there too
Ike