Amazon Echo Dot Vs. Google Home Mini: The Cheap Speaker Showdown
Whilst an important consideration is the hardware (that limits what you can do with regards to sound quality and attachments) another important consideration is the ecosystem you buy into (the backend AI). The former does not change whilst the latter will improve over time. However, on the hardware front, you will also buy other speakers in the same ecosystem range so sometimes the hardware limitations are negated.
In both cases, it comes down to what you want to do with it and what other devices you want to control or what accounts/services you have connected. So in short, there is really no "best speaker" for everyone. The most important thing is for each person to decide criteria is the most important to be considered.