They are actually both undergraduate texts; however, Investments is FAR more complex. Essentials of Investments really waters down the statistical and mathematical notation while Investments does not. Investments also has an entire section (4-5 chapters) called options, futures, and other derivatives while Essentials of Investments does not. [Of course, if you want to learn about options, futures, and other derivatives, there is a seminal book by John Hull with that exact title.]
That notwithstanding, neither book is sophisticated enough to be considered a true graduate school textbook in quantitative investment theory. No grad schools worth their salt are going to rely too heavily on Investments in a specialized finance curriculum. It's a great book to start out, though.