Neither. I know this is an either or question, but f you're using Conkeldurr, it should have Guts. Guts is far superior as an ability for a bulky physical attacker.
If Conkeldurr is burnt, both of its other abilities are no longer a threat.
Iron Fist
>Iron Fist causes the Base Power of affected punching moves to increase by 20%
20% is measly, considering that Life Orb boosts moves by 30%.
Sheer Force
>Sheer Force raises the base power of all attacking moves that have an additional effect by 30%, but the additional effects are ignored.
30% is like a Life Orb boost, but it doesn't boost Drain Punch or Mach Punch, which are the 2 main moves Conkeldurr uses. Sheer Force is more useful on special attackers like Nidoking or Landorus, because burn won't render them useless.
Guts
>Guts raises the Attack stat by 50% while affected by the burn, paralysis, poison, or sleep status ailments. The Attack drop from burn does not occur.
Guts is basically a Life Orb + Iron Fist boost, and it won't make Conkeldurr useless when burnt.
You can either make Conkeldurr carry a Flame/ Toxic Orb, or give it an Assault Vest, considering how common status moves like Toxic, Thunder Wave & Will-O-Wisp are, in addition to Toxic Spikes.
So the Iron Fist + Life Orb boost is exactly the same as a Guts boost, meaning Guts > Iron Fist because of burn and also because you can choose an item other than a status orb.
Conkeldurr @ Assault Vest
Ability: Guts
EVs: 252 HP, 252 Atk, 4 Sp. Def
Nature: Adamant
- Knock Off
- Drain Punch
- Ice Punch
- Mach Punch
Conkeldurr @ Flame Orb
Ability: Guts
EVs: 252 HP, 252 Atk, 4 Def
Nature: Adamant
- Bulk Up/ Ice Punch
- Knock Off
- Drain Punch
- Mach Punch
Conkeldurr
