Dot is better than a damage modifier at ranges past the damage drop of point. Especially when the modifier is only a chance. Thorn will be more reliable at extreme ranges. DoT is less effective at closer ranges where enemies can still do damage. A damage modifier will also work better at mid given the chance of stacking that with a crit will be higher. Also with base damage intact, you won't have to get lucky to be effective as base damage will be high enough for most targets.