We haven't had the chance to try out the AIY kit, but from what we can see online, the difference is that Spencer is based on the ESP32 microcontroller, while the AIY kit is powered by Raspberry Pi.
Spencer also teaches you soldering as you need to solder the components to build it and learn about them in the process.AIY kit connects to Google Assistant and relies on that, while Spencer runs on our servers and has the funny personality we've created.
After you assemble him, you can code him to say what you want, display different images on his LED panel, and customize further. He is a funny little fellow. 😊