We actually recommend using both, hence we created the Combo Pack. The Original Serum is formulated with additional peptides that help encourage lash growth and also contain a stronger percentage of the peptides in the formula compared to the Sensitive. Hence, the result of the Original Serum is more significant, and Sensitive is a gentler formula. Alternating the two serums allows you to maintain healthy, long lashes to a desirable length and fullness. Prolonged use of the Original serum may lead to lashes that reach your brows but like, we are also here for that too :)