First of all, Forretress is the only thing this Hippowdon needs to be worried about going toe to toe with. If you are using a Hippowdon to take down things like Garchomp or Gliscor, then what is the point of the rest of your team? I've said at least once before, this Hippowdon is not a sweeper or a tank, it is to set up Sandstorm and SR, and then get out. Let the rest of your team deal with those common staples, that isn't your job. You are taking data and strategies and misinterpreting them to fit your point; Hippowdon isn't the Garchomp-counter on this team, something like Jirachi or Swampert is.
Second, it is not a contradiction, because I never said that either FF would KO Ferrothorn/Forretress, nor that IF would KO Garchomp/Gliscor. If I have a Forretress and you have a Hippowdon, and your Hippowdon starts using Fire Fang against my Forretress, I cut my losses and switch out. If I have a Garchomp and you have a Hippowdon, and your Hippowdon starts using Ice Fang against my Garchomp, I swat it the hell out of my way and continue my rampage. Fire Fang is a deterrent to those Pokemon, but Ice Fang is not a deterrent to Garchomp; neither one is meant to KO though.
Finally, I think I'm finished arguing. My opinion is that Fire Fang is better. This is not a deterministic problem, it isn't just 1 or 0. You are too emotionally attached to this for some reason, and it isn't going anywhere. Ultimately it isn't my decision or yours, we're just here to offer advice. Smogon is an authority, but they are not the end-all-be-all answer to Pokemon. They just have their opinion, same as us.
As always, still looking for suggestions on my Kyurem. That's the whole reason I've been doing this... <_<