The war of the personal digital assistants got hotter this week. Amazon announced that it's putting its Alexa voice assistant officially on the iPhone. Alexa is available through Amazon's iOS shopping app. It is not yet available for Android.

With Alexa joining Microsoft's Cortana in challenging Siri on Apple's own turf, it brings up the question for consumers of whether it's worth installing multiple voice assistants on their iPhones. Answering that question for yourself is easiest if you know the strengths and weaknesses of each assistant.

Let's start with Alexa. Alexa is most associated with the Echo line of smart speakers, and is best at answering questions, setting timers and performing various skills that users can pick from an ever-growing list of options. Alexa in an Echo can give you a newsflash from National Public Radio, for example, or control smart appliances in your home.

(Amazon's chief executive, Jefferey P. Bezos, is the owner of The Washington Post.)

On the iPhone, however, the assistant's options are currently more limited. For example, I couldn't get a news briefing from the Alexa in Amazon's iPhone app. I couldn't set a timer either. But other features such as getting answers to questions such as -- "Who was the eighth president of the United States?" -- do work. Integration with Amazon services works, too, so you'll be able to ask Alexa to search Amazon.com for the products you want.

Microsoft's Cortana is good for scheduling meetings and reminders that come up during the day. But I've found it's better as a desktop or laptop assistant. On mobile devices, it requires a separate download to get on your phone. That means that you will always have to open the app to use it -- at which point you may as well just type in a search.

Which brings us to Siri and its greatest advantage: it is the default personal assistant on the iPhone. You can summon it with your voice, if you choose that option, any time -- no taps required.

The always-on function of the integrated voice assistant, while a potentially troubling notion for those worried about surveillance, offers convenience. Being able to send a text without looking at your phone, or to set a timer while your hands are covered in cake batter is the true advantage of a digital assistant.

So while there are benefits to installing multiple assistants on one phone, chances are most people will find it easiest to stick to one. Even if all assistants could add always-on voice control, thinking through when you want to say "Hey, Siri" vs. "Alexa!" might offset the time you wanted to save by speaking your request.

They do all tell some pretty lousy jokes, though, if you're in the market for that sort of thing.