For begin, your decision may be influenced by the speaker you prefer visually. Although the Amazon Echo is fairly similar to the Google Home, the Show, Spot, and Dot are significantly different.
The smart assistants built into the speakers are both quite capable. Alexa from Amazon has a variety of skills that set it apart from the competition. However, Google has been a leader in artificial intelligence for a long time, long before the smart speaker market even existed. It’s also much more difficult to adjust Alexa’s voice.