Hominids by Robert J. Sawyer
Two possible worlds connect when the Neanderthals in a world where they were the surviving human race cross over onto our planet. While we have religion and capitalism, the Neanderthals are secular and socialistic. Where we have amazing buildings and huge cities, the Neanderthals keep their population small and thus are breathing cleaner air. There are no easy answers to who has it right, but Robert J. Sawyer presents some interesting arguments in his Hugo Award winning novel Hominids.