Quick take
Use a compact direct-connect under-sink filter when chlorine/taste is the main issue. Use slim tankless RO when broad treatment is needed and cabinet depth allows it. Choose countertop RO only if cabinet access is limited but counter space is available.
Smallest visible footprint: under-sink carbon
Compact direct-connect systems keep the counter clear and can be enough for chlorine, taste and selected certified contaminants.
Tankless RO for broader treatment
Tankless RO removes the bulky storage tank and can fit narrow cabinets better than traditional RO.
Countertop tradeoff
Countertop RO avoids plumbing work but consumes valuable worktop area.