I guess there are a couple of advantages...
As you have mentioned, your possible size for media would be greatly increased, possibly 2/3 fold if you get the right size external.
More space in your tank.
Possible increased surface flow.
Obviously, an external cannister filters take up space underneath in our cabinets, but if you have the room, cash and time to set it up then i would thoroughly recommend one, especially in a tank of your size.
I use the Fluval 205, a filter of the same size and output at the 305 or maybe even the 405 would suit your tank.
Terry.