RO water purifier is an ideal choice for purification of hard water with a TDS level higher than 200 ppm. It is capable of converting hard water into soft water and also remove dissolved solids, impurities, and heavy metals. On the other hand, UV water purifiers should be used for soft water as they are only capable of killing the disease-causing germs and bacteria. They are incapable of removing dissolved solids, heavy metals or the dead micro-organisms.
Thus, RO water purifier is a better option. However, you can also go for a RO+UV water purifier.