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.