The bigger question is what do you need, more power or more defense? I believe Scizor would be a better choice than Steelix. Though Steelix has a higher Defense (by a lot), Scizor can take out opponents faster and has fewer weaknesses. It may not have much of a movepool like Steelix, but its attack makes up for its shallow movepool. Since I don't know which role you want it to be, I'll do both.
Role: Physical Sweeper
Nature: Adamant (+Attack, -Special Attack)
Item: Expert Belt
Iron Head/Bullet Punch - STAB, chance of flinch/STAB, priority
X - Scissor/U - Turn - STAB/STAB, scope out your opponents team
Night Slash/Brick Break - Coverage, somewhat high critical hit rate/Coverage, breaks Reflect and Light Screen users
Swords Dance - Makes him a better Sweeper
Role: Physical Tank
Nature: Impish (+Defense, -Special Attack)
Item: Life Orb
Characteristic: +Attack or +Defense
Thunder Fang/Ice Fang - Coverage, Defense over Water - Types/Coverage, Defense over Ground - Types
Earthquake - STAB, powerful, accurate
Gyro Ball - STAB, compliments its low speed
Curse - Boosts Attack and Defense while lowering speed, help increases Gyro Ball's power (I think).
Hope I helped.
Scizor's ability: Swarm
Steelix's ability: Sturdy