There are 5 factors that you need to consider before buying a water filter system.
Water Pressure: most RO units need at least 45 psi to operate correctly, if your pressure is lower than this, you might look for a RO with Booster pump similar to this Bluonics 5 stage RO with booster pump.
Filtration Quality: Quality of RO filters can be measuring Total dissolved solids (TDS) for the filtered water against the source water.
Flow Rate: Reverse Osmosis has a gallons per day (GPD) number that refers to the amount of water a unit can filter per day at an optimum condition. In most cases, the amount is about 10-20% lesser as stated.
Budget: Before purchasing a RO system, consider both the initial cost to purchase a system and and maintenance costs to replace RO.