Both are widely used streaming methods for audio but can serve different purposes.
Wi-Fi streaming relies on your smart speaker being connected to a wireless network. Wi-Fi offers better sound quality, has a greater wireless range than Bluetooth and allows you to connect to multiple speaker devices, so you can enjoy a full multi-room listening experience.
Bluetooth on the other hand is a short-range wireless solution and works by pairing a Bluetooth-enabled speaker to a smart controller device (phone, tablet, PC....) to access music. This can be a quick and convenient way of streaming when Wi-Fi is unavailable, however the speaker will always be reliant on being wirelessly connected to the smart controlling device. Both devices will need to be physically in range of one another in order to maintain playback (usually limited to a range of up to 10m).