Amazon have been selling the Echo Show smart display for a while now, it combines the Alexa speaker with a screen to allow things like watching prime videos, making video calls and checking smart cameras. You can also do anything that a regular Amazon Echo does. For example control your lighting using voice commands or change your thermostat temperature.
Google has just introduced their own Smart Display competitor in the form of the Lenovo Smart Display, available with 8 and 10 inch screens. Coming soon there will also be Google Home Smart Displays from JBL and LG, with rumors of a Google branded smart display also in the pipeline.
The Google based Smart Displays integrate Google Home and use the screen to display information such as the weather, news, Youtube videos and smart camera video feeds - all accessed by voice commands only. This is not a form of Android tablet but a whole new voice command controlled display.
Choosing a Smart Display may well depend on whether you're already using the Amazon Alexa or Google Home eco-system, to get the most out of the Echo Show you would definitely want to have a Prime account so you can access Prime Video. If you're a big user of Youtube however the Google Smart Displays are currently your best choice - with built in Youtube they work perfectly. Unfortunately Google and Amazon have had a falling out and as a result Youtube is no longer useable on the Echo Show, to the dissapointment (and anger) of many existing users.