When comparing S-voice vs Cortana, the Slant community recommends Cortana for most people. In the question“What is the best intelligent personal assistant software?” Cortana is ranked 2nd while S-voice is ranked 13th. The most important reason people chose Cortana is:
You can ask Cortana what music is playing and she will attempt to identify it.
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Pros
Pro Can read news aloud to you
You can ask S-voice to read the news for you.
Pro Can recognize music
You can ask Cortana what music is playing and she will attempt to identify it.
Pro Will let you know when artists you're interested in play in your area
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.
Cons
Con Wake-up command rarely works
While it's possible to have a custom wake up command be set for S-voice, it more often than not does not recognize the command.
Con Functionality is limited on non-Windows devices
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.