Overall: There's no contest, Ferrothorn is clearly better!
Against each other: Ferrothorn wouldn't stand a chance against Sawsbuck, especially if Sawsbuck had Sap Sipper and Jump Kick.
Even though I answered your question, I guess you'll still want me to compare them huh? Okay then here goes:
Ferrothorn's unique Grass/Steel typing grants it nine resistances to Normal, Water, Electric, Grass, Psychic, Rock, Dragon, Steel and Fairy type moves respectively. It's typing also grants Ferrothorn immunity to Poison type moves, with only Fire and Fighting type moves being super effective. Ferrothorn has both excellent Defense and Special Defense, moderate HP and decent Attack, this allows Ferrothorn to function well as a Support Role, as well as both a Wall and a potentially Tank. Ferrothorn also has a fantastic Ability in Iron Barbs, which when hit by a move that makes contact, the attacker will receive damage equal to 1⁄8 of their maximum HP. If Ferrothorn is holding a Rocky Helmet the damage will stack (dealing 7/24 of the attacker's maximum HP). Ferrothorn does lack an extensive moveset, however it does learn notable moves which include: Power Whip, Gyro Ball, Knock Off, Thunder Wave, Leech Seed, Stealth Rock, Spikes, Toxic and Protect. Ferrothorn acts as an excellent Defensive Wall due to it's fantastic Defense and Iron Barbs Ability, but also because of it's resistance against common attacking moves such as Waterfall, Iron Head, Stone Edge, Rock Slide, Zen Headbutt and Play Rough etc.
What would you say if I told you that Ferrothorn was in OU, while Sawsbuck is in Smogon's newest tier PU, which is basically the lowest ranking tier? Despite Sawsuck being in PU it does have some good features, it's Attack and Speed are relatively good which allows Sawsbuck to function as a Physical Sweeper in PU. It also has okay Abilities in Chlorophyll, Sap Sipper and Serene Grace. Unfortunately though it does have six weaknesses to Fighting, Flying, Poison, Bug, Fire and Ice type moves respectively. It also has fewer resistances compared with Ferrothorn.