These two speakers are the most popular ones on the market. The PLAY:3 is the younger brother of the PLAY:5, they are both great but serve different purposes.
If you plan on purchasing a single speaker and don't want to invest more, we prefer the PLAY:5. It's more powerful, beautiful and well-structured.
The sound of a single PLAY:5 is enough to fill a medium-sized room. As for the PLAY:3, it's also great but you'll need a pair to unlock its full capabilities.
If you're okay with spending more, getting a pair of PLAY:5 speakers is the best decision you can make. With the 'orientate' feature and TruePlay, the experience of the Sonos speakers is out-of-this-world.