Fire Lash is better. Centiskorch has a higher physical attack, plus Fire Lash drops the opponent's Defense, meaning it'll do quite a bit. It does consistent damage, unlike Overheat, which is a one-time nuke. Fire Lash is better in almost all cases, unless you really need a Fire type nuke, which I recommend against. Smogon always recommends Fire Lash, plus Centiskorch gets Coil to further boost its power.
Hope I helped!