I prefer internal for a few reasons. I personally think they can be quieter since they will use the water for cooling while external pumps usually have a built in fan for cooling. In freshwater applications where you are not concerned with excessive heat the internal pump can help with heating of the water although some pumps put out more heat than others.
If you go with internal be sure the pump is isolated from the sump with rubber feet or a rubber mat that will prevent vibration noises. Also be sure the plumbing has at least some flexible tubing for the return to isolate vibrations.
Also internal pumps that develop any leaks will usually be contained in the sump while external will make a mess
Last edited by scubasmac; 03-01-2014 at 01:54 PM.
Reason: Additional info