In addition to what is in the Roman Catechism, the latest CCC also takes into account developments that took place in the Magisterium of the Pontiffs since Trent as well as the last two Ecumenical Councils, as well as Catholic Social teaching and more detailed moral teaching, especially on important issues in our day that did not really exist (or at least were not prevalent) back in the time of Trent.
You will notice in the footnotes that the Roman Catechism (Catechism of Trent) is cited quite often in the CCC. The CCC, however, is broader in scope. It is the most comprehensive Catechism out there.
I use a variety of Catechisms myself: the CCC, Baltimore Catechism, Catechism of Pius X, Fr. Stephen Keenan’s Doctrinal Catechism , Douay Catechism, Roman Catechism, St. Peter Canisius’ Catechism, St. Thomas Aquinas Summa, St. John Damascene’s “Exposition of the Faith”, St. Augustine’s Catechism of Christian Doctrine, St. Ambrose’s “De Fide”, St. Cyril of Jerusalem’s “Catecheticals,” and even the Didache.
They all have their strengths and weaknesses, but the CCC is by far the most comprehensive. I would say if you were going to have only one, choose that one. Next I would add a recent short Q&A type–like the Baltimore Catechism or the Catechism of Pius X. And then if you have room, the Roman Catechism–and then the rest.