Both systems are used by different hot tub manufacturers – which one is better?
Hot tub water needs to be both sanitized (kill bacteria, fungus, etc.) and oxidized (breakdown organic matter). Ozone is a gas (a form of oxygen) which is a very powerful oxidizer and an adequate sanitizer. If it is not used up it reverts to oxygen which means it does not leave a by-product in the water as chlorine or bromine will do. It does however alter the water balance and if there is too much ozone produced it will start attacking the soft plastics – the cover and pillows.
UV light is a strong sanitizer but does not oxidize at all. It does not enter the water and so will not affect water balance or leave by-products. It works by disrupting the DNA of living organisms such as bacteria. Many people find the thought of there being living organisms in their hot tub rather repulsive and so prefer UV light. With a UV light system you will still need to oxidize the water. However you could use a non-chlorine shock such potassium monopersulphate.