For clarity, I'm going to take a pedantic mathematical approach to this question.
If d is the damage dealt by an unboosted HP-draining attack and p is the percentage of this damage restored to the user (in decimal form), then:
Healing with 20% boost items = d * 1.2 * p
Healing with Big Root = d * p * 1.3
1.3 > 1.2, therefore Big Root always causes more HP to be healed than items providing a 20% boost*. This result is not changed by extra boosts (such as from abilities) that apply in both scenarios, because a*b > a*c if b > c.
Big Root is always outclassed by damage-boosting items. This is especially true in the case of items like Life Orb and Choice Specs/Band that do more damage and heal the user more than Big Root as a result.
* Except in the case where damage is low enough that the difference gets lost during rounding. It is more correct to say Big Root heals at least the same amount, but more helpful to say it is always higher.