A homeowner wants to install a basement bar sink, but the nearest plimbing vent is a good 20 feet away. The plumber wants to install a "loop" vent in the cabinet under the sink (like a kitchen island). Would this be preferred over an air admittance valve which may or may not be able to be located with its air intake above the flood level rim of the sink?

Well, it depends whether AAV is legal in your area. It certainly is easier and cleaner way to go with. Also, the highest point of AAV is usually not higher then flood level. We try to install it as high under the sink as possible. However, if AAV is not allowed, than you have to go with complex Loop Vent installation.