The main difference between the lens sizes is the field of view. 

The 9mm has a larger field of view which is good for capturing a lot of area at once, but not so good for getting detailed imagery. 

The 19mm has a much smaller field of view, meaning that you capture less surface area, but have more detail per pixel.  This is generally better for inspections.

Here’s a video link that compares the different lens sizes, and shows the measurements for their fields of view.
https://www.youtube.com/watch?time_continue=5&v=-iDWkGYmUik