Stick with the major brands like Hot Spring, Jucuzzi, Dimension One, Bullfrog, Caldera, ... And stay away from the big box store specials, as they are built to a price point. As to the 120V models, my Hot Spring Jetsetter (2-3 person tub) is a 120/240V convertible model which I have connected for 120V, in 120V mode the heater runs at 1,500 watts, and when the jet pump is running the heater turns off, if wired for 240V then the heater would run at 5,000 watts and both heater and jet pump would run at the same time. Overall operating in 120V mode works ok for me, the down side is that the water cools off a little (1-2 degrees as the jets do help maintain some heat) while the jets are running, but this is usually not a big deal with the way I use the tub which is mainly for hydrotherapy for my back. If I were to use the tub for hours at a time with the jets running as a more social tub, then I would want to wire it for 240V in order to maintain a constant temperature. Overall even in 1,500 watt mode the heater does ok, it just takes longer to warm up the water after a refill (12-18 hours).
Ike