It's too easy, almost lazy, to lump Apple's Siri and Amazon's Alexa into the same category. They're both virtual personal assistants made by two of the biggest, most influential companies in tech. They both answer questions, add things to shopping lists, control your smart lights, and tell awesome dad jokes.
But in reality, Siri and Alexa are pretty different; in fact, sometimes it seems as though the only thing they have in common is that they both respond to voice. Siri is on Apple mobile devices (and Apple TV); Alexa is on speakers (and Amazon Fire TV). Siri is tightly controlled by Apple; Amazon has welcomed developers to the Alexa party by allowing them to create all kinds of "skills." Siri needs to be close to you; Alexa can hear you from across the room. And that's just the start.
Right now the biggest difference is probably how they're meant to be used. Siri is with you in your pocket, in your car, on your wrist, while Alexa works best in the home, or at least, somewhere with a very stable Wi-Fi connection. But at the same time, Apple is trying to make Siri more accessible in the home, while Amazon has actually made Alexa portable. So, in regards to the question, "Which is better?" — right now that's still somewhat dependent on use case. But soon it might not be, as these virtual assistants become an essential computational layer over everything. I know one thing: I certainly use one of them much more than the other.