The Holy Spirit guides each of us to pray, and I believe that would also include the particular form of devotion. Since Our Lord Himself taught the Chaplet of Divine Mercy to St. Faustina, we have no fear that it is less important a devotion than the rosary, which was given to us by His Mother. Both have their purpose, both have their interior unction and attraction.
I pray the Chaplet of D.M. for special Souls in Purgatory, as Our Lord instructed Faustina; however I pray the rosary for the conversion of my family members who are not in communion with God.

We also know from the Diary’s revelations that the Chaplet of D.M. is to be prayed for those who are dying, as it is a most effective means for them to obtain mercy at the hour of their death.