A top-mount sink is one that is installed on top of a countertop. A portion of the sink, called a lip, overlaps the countertop to provide a covered edge.
This type of sink is set up below the actual countertop, yielding a smoother finish. There are no extra grooves or crevices that will trap dirt and debris.
An under-mount sinks gives a cleaner look. It allows the countertop selection to be showcased and functions better during cleanup.
Top-mount sinks simply fit into a cutout already made in the countertop, and a faucet can easily be installed afterward. Under-mount sinks require additional support by a brace or frame, because they are positioned underneath the countertop; the placement of a faucet has to be secured underneath as well, hindered by a small amount of working space.
Under-mount sinks tend to be more expensive, because many are custom built. and the labor is much more extensive than a top-mount installation. Under-mount sinks are typically used in kitchens with natural stone countertops; therefore, the cost of granite or marble is likely to be reciprocal in the selection of the sink.