cp from other thread
The reason people compare Ferrothorn to Forretress is because they're both commonly used as spikers. The reality is they're two completely different Pokemon who just happen to share a type, a couple moves, and a double weakness to Fire, which is irrelevant because that's what you've got five other Pokemon for.
One of the biggest things separating Ferrothorn from Forretress is his resistances. Forretress has a welcome Ice resistance, while Ferrothorn has extremely valuable resistances to both Water and Electric, which are arguably more important resistances to have.
When you look past the fact that they can both learn Spikes and Stealth Rock, you'll find that they fulfill vastly different roles. For a purely supportive role, Forretress is superior due to access to Rapid Spin and Toxic Spikes. He also has slightly better physical defense than Ferrothorn.
In every other aspect, Ferrothorn wins; his Attack stat is slightly higher and he's literally twice as slow as Forretress, making it easier to get 150 BP Gyro Balls. He has almost twice Forretress's Special Defense (116 vs. 60), which makes Ferrothorn better at absorbing Hydro Pump and Draco Meteor than Forretress could ever dream of due to almost identical HP stats. The fact that he takes neutral damage from Ice is almost a non-issue due to the base stat difference. With investment, making Ferrothorn shrug off Ice Beams is easy.
Unlike Forretress, who is widely seen as setup bait, Ferrothorn can put pressure on the opponent with access to Curse, Leech Seed, Thunder Wave, Worry Seed, Swords Dance, and Power Whip; the latter of which is very important because it gives Ferrothorn an alternate and powerful STAB option, something Forretress doesn't have (lol Bug Bite).
If you put Ferrothorn on your team, play to his strengths. He can function as a spiker, but only if your team needs his valuable resistances. People use him the same way they would Forretress only to find that - surprise - he sucks. Ferrothorn actually gets set up on by Forretress because Forretress can spin away anything Ferrothorn throws at him while resisting both of Ferrothorn's STABs (Grass 2x), which should go to show you who is king of pure support.
In a nutshell, use Forretress for defensive support and use Ferrothorn for offensive support.