Google Now automatically pulls information from Gmail about upcoming flights and lets you know if the flight has been delayed, how long before a flight you should leave to get to the airport on time, offers hotel options, informs of relevant currency exchange rate, as well as suggest places to vis...
Cloud-based speech synthesis is nearly indistinguishable from a real human. Also, starting from Android 7.0 Nougat, users can alter the pitch of Google Now's voice. Studies have shown that higher-pitched female voices are considered more attractive by the majority of the population.
Sometimes, when you have a poor internet connection, or Google is being lazy, you'll get the on-board speech synthesis instead of the silky-smooth cloud-based speech synthesis. It's very good, but not nearly as good as the cloud-based voice.
While Cortana is available on Android and iOS, certain features on those operating systems are either limited or non-existant. For example, there's no hands-free voice activation on iOS, and on Android it's only available on the homescreen and nowhere else, etc.