The Book Of Souls, imo, is a better album. More varied and heavier. The Final Frontier was good, but except "El Dorado", "The Talisman", "The Man Who Would Be King" and even "When The Wild Wind Blows", I wasn't really into it. TBOS has this proggy and heavy vibe from the 80's, especially the one from Powerslave and the title track "The Book Of Souls" feels like "The Rime Of The Ancient Mariner" on the strong and muscle parts. And on this record there are some great tracks like "If Eternity Should Fail", "Speed Of Light", "When The River Runs Deep", "The Book Of Souls", "Death Or Glory", "Shadows Of The Valley", "Tears Of A Clown" and "Empire Of The Clouds". So, for me, it's clear that TBOS is more balanced and more diverse than its predecessor.