Ferrothorn doesn't get Toxic Spikes, and it is weak to the most common offensive type in the game.
Forretress is also weak to Fire, just so you know. --- Chuck Finley is Forever! People that agree: 5. I believe in Jesus Christ as my Lord and Savior and am 100% proud of it. Yeah, it's hot, but it's a dry heat.
Forretress can lay spikes/toxicspikes/stealthrock, isn't weak against fighting, can rapid spin, and Sturdy pretty much promises it'll at least get one move in. Also, it can use Volt Switch to escape trappers.
Ferrothorn can lay spikes/stealthrock, has almost double Forretress's base spdef, resists water, can SubSeed, is much better offensively, and can punish contact attacks with Iron Barbs.
1 on 1, Ferrothorn wins easily with leech seed and Iron Barbs(against rapid spin), but it really just depends on what you need. --- Feel free to apply sarcasm tags if you didn't like my post.