Personal digital assistants, rise up!

FORTUNE — The arrival of Cortana, Microsoft’s (MSFT) digital assistant for its Windows Phone mobile operating system, means there’s another decision to be made when it comes to buying a new smartphone: Who do you want running your life for you?

The newly unveiled Cortana app competes directly with Apple’s Siri (AAPL) and Google Now (GOOG) for the chance to organize your appointments, contacts, travel arrangements, and all of the other minutiae of daily life. Microsoft calls Cortana the “truly personal digital assistant” — a thinly veiled dig at its forerunners — that analyzes how you use your phone to learn what topics you’re interested in, when you’re busy, and where you travel.

Cortana can deliver a summary of relevant news stories at the start of the day. Like a flesh-and-blood personal assistant, it also allows users to set up a “quiet time” where calls and texts are silenced as well as letting an “inner circle” of contacts break through those restrictions. The Notebook feature lets you tweak what the app knows about you, providing a more granular level of control than either Siri or Google Now. Cortana seems to have a sense of humor too. Tell it to open the pod bay doors on your space ship, and she’ll reply that she’s afraid she can’t do that, Dave (a sly reference to the sci-fi film 2001: A Space Odyssey).

In many ways, Cortana combines Siri’s voice search with Google Now’s suggestions, which are based on data like your location, the time, and your appointment calendar. Some early reviews say Cortana is a step ahead of Apple’s and Google’s services, mostly by copying the best features from each. Cortana can tweak a phone’s settings like whether to connect to Wi-Fi. (Google Now can’t.) Cortana can also track a flight’s status. (Siri simply returns a web result.)

For now, only a test version of Cortana is available in the United States.

Cortana’s competition: Siri

Siri started the digital assistant trend as an independent, stand-alone app before being acquired by Apple in 2010. A year later, Apple integrated Siri into iOS and started marketing the service as an “intelligent assistant that helps you get things done just by asking.” Over time, Siri has become better at recognizing and interpreting commands while linking to more apps in Apple’s ecosystem. What it doesn’t have is all of the personalization and data collection found in Google Now and Cortana.

Siri plugs into your emails, contacts, and calendar, but it’s not trying to find out everything about you. “In some ways, the approaches each company has taken in the space is a reflection on their heritage,” says Tony Costa, a senior analyst for Forrester. “Google Now is much more like a search engine — efficient, task focused, and trying to anticipate what you need. Apple Siri is much more of a natural language voice user interface. And Cortana a bit of a blend of the two approaches.”

MORE: Satya Nadella needs more than one trick to fix Microsoft

Currently, Apple does not have the breadth of services for Siri that Google and Microsoft have, and that’s an area of concern for the company, according to Daniel Gleeson, an analyst for IHS Global. “Apple is relying much more on outside technology,” he says. Voice recognition from Nuance and search results from Bing are just a couple of examples of help it’s getting from others. “Google does all of that itself, while Microsoft has been researching voice recognition for over 20 years and has its own search engine as well.”

Despite being first with a digital assistant app, Apple is suddenly playing catch-up, says Gleeson. “You can actually see Google and Microsoft moving ahead in this space,” he says. “Many people will say that Google Now and Cortana are already much more useful than Siri. Apple does need to address that, otherwise it will fall behind, and that’s not a position that Apple is used to being in.”

Siri’s voice control and personal data gathering were a first for mobile phones, but it’s no longer enough. For a truly smooth and personalized experience, our digital assistants need to know as much about us as possible, and no one does that better than Google.

Cortana’s competition: Google Now

Google Now didn’t premiere until 2012, but it has quickly become a critical feature in the company’s Android operating system. Like Siri, it can be activated and controlled by voice, but voice isn’t front and center the same way it is in Siri or Cortana. Instead, Google emphasizes the app’s ability to predict what information users need and when. Recent searches, regular travel routes, and email messages are all scanned for information the user might want to see straight away — the delivery status of your Amazon order, the weather, the latest news about a favorite sports team. In short, it doesn’t wait for you to ask.

While you can still talk to your phone to “send an email” or “wake me up in half an hour” as you can with Siri, Google Now adds suggestions and recommendations based on your searches, activities, and other data. Spend every Monday evening at the gym, for example, and it will soon learn to show directions on cue every Monday. This is something Siri doesn’t yet have, and which Cortana is trying to copy.

MORE: Nadella: Microsoft needs a ‘data culture’

Microsoft is hoping that the privacy controls it has built into Cortana will provide a more appealing product for consumers than the all-pervasive Google Now. The Notebook settings page that lets users set exactly where Cortana can and can’t look is crucial. “Probably the key difference between Google Now and Cortana is that easy control over what is displayed and what apps Cortana has permissions to look into,” says Gleeson, of IHS Global. “Microsoft positions itself against Google by saying, ‘We don’t look into your private stuff,’ so this really continues that way of looking at it.”

While the competing digital assistants offer a lot of similar functionality (creating appointments, searching the web), there are fundamental distinctions: At its core, Siri offers an alternative way of interacting with your phone and some of the apps on it. It’s good at understanding natural language and acting on your instructions, for example. Google Now and Cortana add a more comprehensive collection of services and try to replicate the functions of a real-life assistant.

Left to right: Apple’s Siri, Google Now, and Microsoft’s Cortana

Beyond the smartphone

Another crucial differentiating factor for these digital assistants are the ecosystems they’re plugged into beyond the mobile phone. A personal assistant can only be truly useful if it follows us everywhere. This is why Microsoft, Apple, and Google are working to bring their respective services to many more devices, and not just phones and tablets.

In March, Google announced that Google Now’s information and suggestions would appear in the Google Chrome Web browser for users who were signed in. Cortana, meanwhile, has already appeared on the Bing homepage. As for Siri, it’s set to be the cornerstone of the new CarPlay platform that brings iOS to automobile dashboards. Where can these companies expand next? Desktop operating systems and search engines are likely candidates as are video game consoles and television sets. Microsoft’s Xbox can already be controlled by voice, for example. And that’s to say nothing of the nascent market for wearable computers, of which Google’s Glass is a part. On small, limited displays, presenting the right information at the right time will be crucial.

In all of these channels, the digital assistant serves as a facade. In exchange for helping with our lives, technology companies receive endless streams of personalized data about us. That’s not just good for business, it’s also useful in helping the service continue to improve. At least, that’s the argument given by companies that are promoting digital assistant technology.

“It’s about sharing the data and the same performance across multiple platforms, whether it’s a laptop, Xbox, or tablet device,” says Francisco Jeronimo, research director for IDC. “What’s important for me as a user is bringing everything along the way in a very easy way, and having the content available in different places. Having the same experience at my desk or on a tablet or in the car when I’m driving … that’s really powerful and extremely valuable for users.”

Subscribe to Well Adjusted, our newsletter full of simple strategies to work smarter and live better, from the Fortune Well team. Sign up today.