The first was listed as a standard text for the core physical chemistry courses (CHEM 260 and CHEM 360 at SFU). It covers most of the physical chemistry you will need for a B.Sc Major in Chemistry. The second text covers only the CHEM 260 part of physical chemistry and is therefore not so heavy.  In fact, Engel and Reid also publish. a full Physical Chemistry text, which includes all the chapters of the smaller book as well as the additional material needed for CHEM 360.  It is your choice whether to buy a full physical chemistry text or a smaller book which just covers this course.   You do not need both.  You do not even have to buy one of the "required" books.  I (Dr. Percival) don't teach or assign problems from a book, so an older edition or an alternative text should  be sufficient. Any standard physical chemistry text should do, as long as it is aimed at Chemistry Majors. There are also many specialist texts on quantum chemistry (e.g. an excellent text by McQuarrie), but they typically introduce topics in a more mathematical way than CHEM 260.