Okay I called Panasonic and talked to a nice girl in the Philippines. She didn't know anything about bathroom fans but was able to tell me that the WhisperGreen line is rated for continuous use and uses DC motors. The WhisperCeiling line uses AC motors and is rated for "spot" use. The Dimensions and the installation are the same for both.
Not sure if there are any other differences, but I ordered the FV-08VK3 WhisperGreen it was $15 more expensive than the WhisperCeiling. Hopefully that was the right choice.