Obviously it varies on setup, previous experience, etc. But I have never used a sump that I would prefer over a Fluval FX5.
There may be more room for media, but the media isn't used as effectively. Sumps can be inefficient mechanical filters. As soon as the media is clogged at all the water will take the path of least resistance and simply go around the mechanical media. In a canister this is not an option. As the media collects more debris there is no path of lesser resistance and the water keeps going through the media and keeps getting cleaned. You only need so much biomedia. If the tank only requires one liter of submerged biomedia it is of no benefit to have a sump with the ability to hold ten liters.
I know many people use and prefer sumps over canisters, but IME a canister is a more effective, easier to setup and maintain, and usually cheaper option. All I want to do is make sure that people at least look into canisters when deciding on filtration.