While I am in no position to give a reflection on the differences or main points even of each of these books, I wanted to share a little word on the purpose of the two books for those not familiar with them.
Introduction to the Devout Life
This book was written by Holy Father for lay people.
Treatise on the Love of God
In his second work, Holy Father addresses the individual who has already begun the spiritual journey and wishes to go deeper. It speaks more of prayer and the relationship with God. how to conquer temptations and distractions, how to approach different levels of prayer, spiritual aridity, etc. Some say it is the Salesian form of St. Teresa of Avila's Interior Castle.