Do you need RO filtration?
RO filtration is appropriate when: your water source has documented heavy metal levels (lead, arsenic, chromium) above EPA action levels; your TDS is above 500 PPM; you have fluoride concerns; you're in an agricultural area with potential nitrate contamination; or your water source is a private well. For municipal water where the primary complaint is chlorine taste and odor, a carbon filter (Waterdrop N1, Aquasana Claryum, Brita) is adequate and costs less.
Countertop vs under-sink RO
Countertop RO (Waterdrop D6): no installation beyond faucet adapter, works for renters, lower capacity and flow rate. Under-sink RO (Waterdrop G3P800): requires installation, dedicated faucet hole in sink, higher flow rate (800 GPD), smart monitoring, better suited for primary kitchen filtration in owner-occupied homes. Choose countertop for flexibility; choose under-sink for permanence and capacity.
Waste water consideration
All RO systems produce waste water. Waterdrop has improved the ratio significantly (D6 at 1:1, G3P800 at approximately 1.5:1) versus traditional RO (3:1-4:1), but if zero waste water is the goal, a Berkey gravity filter is the correct alternative -- it removes bacteria and viruses without any waste water and without electricity.