I personally prefer Rocky Helmet, as one of Ferro's weaknesses is Fighting, allot of people will use Close Combat / Dynamic Punch etc. to try to bring it down, to weaken physical sweepers by 25% + entry hazard damage can be crucial to blocking a sweep attempt. Also, allot of the fire types these days use physical attacks such as Flare Blitz or Fire Punch, so the helmet can come in handy there, too.
Big Root is also nice, but there are so many Magic Bouncers / Magic Guard / Taunters / Grass types that will try to nullify Ferro's seeds, that it isn't worth sacrificing the item slot for one easily countered move. Iron Ball is cool with Fling, and its Gyro Ball synergy, but Ferro is already deathly slow, so there isn't much point.