They have totally different niche. Ferrothorn typing is better for walling special attackers, while Chesnaught looks like a Pokemon which is designed to take physical attackers. As I said before - Grass/Fighting is great for it. Resisting EdgeQuake combo, which many Pokemon use for coverage is brilliant to have and you have some other great resistances (Dark, Water, Grass to name few), make him good at this job. I checked calcs, he 100% walls Excadrill and standard Landorus sets, no matter what they do. Also takes every single hit against Calm Mind Sacred Sword Keldeo and Choice Specs (just watch out on Focus Blast and Icy Wind, everything you take fine without SR up, it's ironic how Sacred Sword hit him on his much better physical bulk). If you want to be 100% sure to check Keldeo and Gengar as well with Chesnaught, I would recommend to change nature from Impish to Careful. You don't need this positive nature on defense, as you still 100% wall Excadrill, Gyarados and Landorus with standard movesets (while you still 100% counter CB Tyranitar with Stone Edge/Crunch/Earthquake/Aqua Tail and just watch out on random Fire Blasts/Ice Beams), while Careful nature allows you to avoid 2HKO from Keldeo Specs Hydro Pump with SR 100% of time, which is good. I'll edit this post later with calcs.