I would not consider a canister (any kind) as the best choice for the discus tank, unless one plans on taking it apart very (and I mean very) frequently for cleaning. Because doing it frequently enough is unlikely, HOB filters would be my preference in that situation.
I love FX filters. I have two FX6, one in a 125g and another in a 75g. I service them frequently and both have pre-filters. However, I would not feel I can service them frequently enough for a thriving discus tank. I did have discus for years. JMHO. Good luck!