This depends a lot on your preferences. Android users who are already used to Google Assistant often prefer Google Assistant devices. Plus, Google's smart displays fully support YouTube, YouTube Music, and YouTube TV, while Amazon's displays only offer these in a browser.
Another difference is Amazon Alexa displays require both parties to have either an Echo Show device or the Alexa app for video calls. Google Assistant works with other Google displays, Duo, Meet, and Zoom.
You'll find plenty of users for both that claim one or the other is better at voice recognition and getting commands correct. Neither one is perfect at it. I'd suggest downloading both the Google Assistant and Amazon Alexa apps on your smartphone and testing them both to see which you prefer.