You can't over filter. the good thing about having both is that the FX5 is a huge mech. filter and the wet/dry is a huge bio fiter. They both can have a good amount of chem. media allthough IMO chemical media is not that nessisary if ou keep up with your water changes.
So I would go with both but look into a different wet/dry you should be ableto find a good used onearound here somewhere