Internal filters are easy to install, trouble-free and easy to maintain. Filter material can be quite small and may often need cleaning. Internal filters are ideal if you have an area in your aquarium where water flow needs to be increased, or if your tank carries a high bioload and is in need of additional filtering.
An external filter usually has a much larger filter volume than an internal filter. In an external filter it's possible to use a wide variety of filter media in contrary to an internal filter. If the filter has to been cleaned, you do not have to go into the aquarium, which causes less stress for the fish in the aquarium.