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The fx5 is a name brand filter, but the sump is more costumizable. I would choose the fx5 because it takes up less space, but it probably doesn't have as good of flow than a sump.
Both? You won't go wrong either way... ultimately, it's up to you to decide whether you want the sump that can add volume and get your heater out of the tank (plus you could grow plants in a chamber for nitrate control), or a canister that's easier to set up, smaller, but will take a little more time for maintenance.