It is a matter of what best “fits” your application as well as your equipment preference.
A mechanical skimmer will remove the tramp oil layer via a pickup mechanism, such as a disk, belt, or tube. The tramp oil is then discharged automatically to a waste container, which will need to be manually emptied as needed. Since tramp oils are best removed daily, as they are a food source for bacteria, it is recommended to have one on each machine sump to run as needed.
A coalescer draws in the entire surface layer of your sump, then gravity separates the oil within its holding tank for manual discharge to a waste container. Again, oil removal should be done daily, so dedicate a coalescer (such as a Muscle) on each sump to run continuously, or daily utilize a portable unit (such as the Snapdragon) on each sump.
More information on each product selection can be found in their product sections.