In many cases, Essentials!
For students who have completed Foundations, Essentials is an excellent next step whenever the student is ready to go deeper with spelling mastery and grammar.
Some young children who have finished Foundations may not be ready to think more analytically about spelling yet and may be better served by spending a year focused on reading, using children's books or a literature program of your choice. You can keep their spelling skills sharp by playing the occasional phonogram game and analyzing some words from books they are reading while focusing primarily on building reading fluency and comprehension skills.
In Essentials, students review the phonograms, spelling rules, and linguistic concepts they have learned in Foundations, and the emphasis shifts to applying them for mastery in spelling. They also learn the remaining spelling rules, new morphology and language tools that further strengthen vocabulary and comprehension, and foundational grammar concepts such as the parts of speech.
The three levels of instruction within each lesson allow you to choose the level of spelling words, grammar practice, and vocabulary activities appropriate for your student. Students coming from Foundations will often be able to skip the beginning level activities and use Level B or C instead. The vocabulary instruction sections for the Level B and Level C spelling words introduce almost 200 new roots, prefixes, suffixes from Greek, Latin, Old English, French and more.
In addition to using Essentials for spelling, grammar, and vocabulary, students coming from Foundations should read widely using whatever literature or reading curriculum you prefer. Having learned the phonograms and spelling rules they need to decode 98% of English words, they are ready to begin exploring whatever books are the best fit for their interests, age, and vocabulary level.
For a detailed comparison chart outlining the similarities and differences between the two, see What's the Difference Between Foundations and Essentials?
If you are new to Logic of English and trying to decide between the two programs as a starting point for a particular 7, 8, or 9-year-old, see Do We Want Essentials or Foundations? on the Logic of English Blog.