In short: Spencer is a kit you solder together and then program yourself, and it runs on CircuitMess's own servers, while Google's AIY kit is built around a Raspberry Pi and hands the listening and answering to Google Assistant. Spencer is no longer sold, so this comparison is for people who already own one.
The hardware. Spencer is based on an ESP32 microcontroller. The AIY kit is powered by a Raspberry Pi.
The build. Spencer is a soldering kit. You solder the components together as you assemble it, which is much of the point of it: you can see what each part does as it goes in.
The voice. The AIY kit relies on Google Assistant for what it hears and what it answers. Spencer runs on our own servers and has a character of its own, so how he responds is ours rather than Google's.
After the build. Once Spencer is assembled you can code him: change what he says, show your own images on his LED panel and alter how he behaves.
We have not tested the AIY kit ourselves, so the AIY points above come from Google's published material. Spencer's build guides are on our Resources page.