Your raw water quality
You should always select your product depending upon your raw water quality.
Water quality if often measured by TDS (Total Dissolved Solids). If the TDS of your raw water is high, that means that the water contains Iron, Calcium, Magnesium etc. in high volume.
If the raw water TDS is over 500 ppm., you may prefer a RO water purifier. If the same is below 500 ppm. you may consider buying any UV water filter.
Your water consumption
If your raw water TDS is below 200 ppm. and your water consumption is around 10 liters a day, you may buy any good non-electrical water filter.
In case your water requirement is more than 60 liters per day and the raw water TDS is below 300 ppm. then ideally you will need a good UV water purifier.
Because the production rate of RO systems are slow, you can go for RO water purifier in case of high TDS and low water requirement.
Technology of water purifier you are using
In terms of purification RO is the best technology. Where the purification level can be checked with simple gadget.
Brand you are using
There are lot of Brands available in India. Locally assembled machines are available every where. But we suggest to buy branded products. You will get proper after sales service from branded products. These products need frequent service and maintenance, so after sales servicevery much important.