It depends what you want for your team & also which tier you're playing in (the usual answer for these kinds of questions). I'll add the EVs for the sets as well, even though you say you've already EV trained.
Escavalier
Massive attack stat of 135, very decent defences & good typing, but slow speed & mediocre HP, along with not very appealing abilities.
Role: full out attacker
Escavalier is only able to be a full out attacker, so if you need a good sweeper and some bug coverage, go for Escavalier. It can work very well on Trick Room teams, due to its low speed guaranteeing it to go last, and its typing means that the ever Popular Mach Punch is x1. Choice Band is a common held item, adding a further boost for an already monstrous attack stat. However, while Magnezone is not seen in RU/ UU, Escavalier does need to deal with Magnet Pull Magneton, and one HP fire will end it. Unlike Scizor, Escavalier is even slower, and has nothing to deal with Magneton, its only viable fighting move being Reversal.
Escavalier @ Choice Band
Ability: Swarm
EVs: 252 HP, 252 Atk, 252 Sp. Def
Nature: Adamant
- Megahorn
- Iron Head
- Pursuit
- Sleep Talk
First two for STAB, second to deal with psychic types that try to escape, and lastly to be a status absorber.
Ferrothorn
Good typing, useful ability, immunity to leech seed, access to many hazards & status moves, very good defence stats, but slow & lacking in HP.
Roles: staller/ wall/ hazard set up || attacker
Ferrothorn can serve a few roles on your team, the most common being a hazard set up & Leech Seed wall. The less common use for Ferrothorn is an all out Choice Band attacker, since it does have some very good STAB moves in Power Whip & Gyro Ball, benefiting from its slow speed. It's especially popular on Rain teams since HP Fire will be less likely to OHKO it, as well as providing a Grass/ Water/ Electric resistance. However, once again slow speed will mean it has to go last unless under Trick Room, and its typing means it's weak to Fighting as well (unlike Escavalier).
Ferrothorn @ Leftovers/ Rocky Helmet/ Shed Shell
Ability: Iron Barbs
EVs: 252 HP, 92 Def, 164 Sp. Def
Nature: Relaxed
- Leech Seed
- Thunder Wave
- Spikes
- Power Whip/ Gyro Ball
Shed Shell is useful in this scenario, since in OU Ferrothorn will need to deal with Magnezone very frequently.
Ferrothorn @ Choice Band
Ability: Iron Barbs
EVs: 252 HP, 252 Atk, 4 Sp. Def
Nature: Brave
- Gyro Ball
- Power Whip
- Bulldoze
- Explosion
However, Escavalier's Choice Band set outclasses Ferrothorn (while still having a respectable attack stat of 94). If you want an all out attacker, go for Escavalier.
Conclusion - basically, Ferrothorn for some resistances & hazards, Escavalier for some Trick Room sweeping. If you don't use Trick Room, Escavalier will have to take a few dents before it can do anything. In OU, the better sweeper is Scizor, since it has the ever useful U-Turn to escape from a Magnezone trap, or Superpower to deal with it.
