Hi Garth, welcome to MASA.
A sump is used to house all the equipment that you don't want to see in your display tank, heater, skimmer, space for calcium, phosphate and carbon filters along with a DSB, it also provides a larger water volume for the benefit of your display tank which helps to lower the dangers of chemical issues, the only "cons" i can see with a sump, room to house it and the possiblilty of a flood due to blocked syphons or power loss.
External cannisters, do their job if you have little room, you can run phosphate and carbon, as well as filter floss.
If you have the room, go for a sump, the pro's outweigh the cons.