Simmel's engagement with and criticism of Schophenhauer and Nietzsche here is admirable. Although I have not read much secondary literature on Schopenhauer, I HAVE read piles of secondary literature on Nietzsche and this ranks right up there with Deleuze's classic "Nietzsche and Philosophy." Here, Simmel is primarily interested in how both thinkers' teachings can be applied to the problems of life as life is actually lived (yet without falling into shallow, narcissistic self-helpism). This makes for a refreshing approach, and I wish there was more secondary literature written in this style.
Simmel has some sharp criticism for Schopenhauer's rigid, dogmatic pessimism, yet acknowledges the beauty of Schopenhauer's system, which was built up from a variety of respectable sources of inspiration including but not limited to: the (Hindu) Upanishads, Plato, and Kant. The journey of Schopenhauer's philosophy is fantastic, but the destination is rotten. Schopenhauer makes some wonderful points which seem to imply the sublimity of life and the cosmos, yet ends up renouncing the will-to-live, and making out human life to be a lousy, contemptible thing.
Nietzsche, as Simmel highlights, attempted to affirm life, using Schopenhauer's tools towards a better conclusion. But he quickly got in over his head. Life is too chaotic, too intense, too tragic to affirm in any straightforward way. If Schopenhauer was a dogmatic pessimist, Nietzsche was a dogmatic optimist (see "The Gay Science"). Even if Nietzsche's philosophy takes some disturbing turns, those always occur in the context of his overarching project of affirmation.
I think Simmel's basic assessment of both thinkers is correct. He managed to capture the essence of both by playing their thought off of one another, as if they were in dialogue, arguing over the many problems in life. Ultimately, Simmel may have found a better answer to the problems grappled with by both previous thinkers. That answer is presented in "The View of Life," a deeply revelatory work which deserves far more respect and attention than it has received.
Unrelated: As others have wondered... What the heck is going on with that cover art?