Apple's Siri, introduced as the big new feature in the iPhone 4S, is facing its biggest competitor to date in Samsung's S Voice, an extremely similar voice-activated mobile assistant that will make its retail debut with the Galaxy S III early next week. In anticipation of that time, I've brought the two together for a side-by-side comparison to determine which performs better at the daily tasks one might wish to entrust to a robot-voiced virtual assistant.
The first thing to say is that neither Siri nor S Voice is particularly good. Both demand that you enunciate studiously if you care to have your query recognized, and even then some hilarious misinterpretations can and do occur. That's a big stumbling block for any piece of software that aims to streamline your user experience with a smartphone — if you have to repeat or correct yourself, you might as well use more conventional means to achieve your goal. Similarly, both Siri and S Voice have a tendency to rely on external search engines for their results, sometimes integrating them in stub form within the app and sometimes throwing you out to a Google search. S Voice does the latter quite a bit more often than Siri does.
In terms of speed, Siri has a distinct advantage over S Voice. It consistently provides an answer ahead of its Samsung competitor. On the other hand, S Voice can actually perform geographical searches whereas Siri seems to think everything I search for is within the UK and refuses to try and help with locational queries. Siri is also a lot more vocal than Samsung's application. The majority of queries sent to the iPhone are greeted with an aural affirmation of some sort, whereas S Voice tends to provide results quietly (and also includes the option to turn off audio responses altogether). Which of the two approaches you prefer will be a matter of personal preference.
Siri is faster, S Voice can do more, but neither is really good enough
Taken in totality, I would say that S Voice offers a very good approximation of what Siri adds to the iPhone — helping you take notes, look things up, schedule meetings, or check the weather — but that extra functionality isn't yet useful or polished enough to make these voice assistants a real selling point for new phones.
Technical notes: Both the iPhone 4S (Siri) and the Galaxy S III (S Voice) were connected to the same mobile network (Three UK) and to the same Wi-Fi network, the latter providing a 10Mbps connection.