Quick take
Choose under-sink for permanent high-flow kitchen use and less counter clutter. Choose countertop for portability, rental housing and no-install convenience.
Installation
Under-sink systems usually connect to the cold-water line and may use a dedicated faucet or direct-connect setup. Countertop systems can be refillable appliances with no plumbing changes.
Flow and capacity
Under-sink systems generally handle more continuous household use. Countertop systems are limited by feed-tank size or appliance flow.
Portability
Countertop wins for renters and frequent movers. Under-sink equipment is better for homeowners willing to install once and use for years.