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 visit when on location.
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.
Cortana monitors your Bing search history, social network actions, music played, etc to find out what are your favorite artists and will let you know if they plan to play in your area.
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.