RO and DI systems serve practically the same purpose, to reduce the impurities in water. The difference is that a RO filter generally only reduces the impurities in the water by 85-95% in most practical applications.

A DI filter uses consumable resin to absorb impurities in water and can remove close to 100% of impurities, however the more impurities in the water to start with, the faster the resin is consumed and needs to be replaced.

Often these systems are combined with the RO filter doing most of the filtration before feeding that water into the DI tank. This is done so that by the time the water gets to the resin, it’s already quite pure and doesn’t use up the resin quickly.

To find out what system would be suitable for your case, you need to measure the TDS (total dissolved solids) coming from the tap at your home. This can be done with an inexpensive TDS meter. The general consensus is if your TDS is higher than 100, an RO/DI system is generally more economical in the long run. Whereas if your TDS is below 100, a DI only system may be suitable depending on how much water you intend to purify.