I use both filter types described and both scenarios (2 x XP3 or 1 x FX5) would be anticipated to produce approximately the same output of approx. 400 gph...give or take, when filled with media. I run 2 XP3s on a well-stocked Dat tank with no carbon, only mechanical and biomedia and they perform beautifully. I have no reason upon which to believe that you will see any difference in terms of water quality and either scenario will cost approx. $200.00. Therefore, I would suggest that you consider other filter-specific features which may have applicability given your individual preferences.