Courtesy of Justin Fore at Fore Innovations LLC:
Both pump types are capable of very large fuel deliveries, but external pumps require significant fuel line plumbing under the vehicle, which leads to higher possibility of leaks and/or damaged lines or components. Conversely, an internal pump setup places the prefilters and the pumps inside the tank, leaving only fuel lines and a single filter outside the tank. Generally speaking, external pumps are more reserved for race only applications, since race cars have requisite fire suppression systems and operate at a track environment, where fire supression personel and equipment is readily available. Additionally, internal pumps are more quiet than external pumps.