While the legacy CLI (versions 0.18 and below) is still available, it is not receiving any non-critical updates and it is recommend migrating to the new CLI as soon as possible.
"Databricks recommends that you use Databricks CLI version 0.205 or above instead of the legacy Databricks CLI version 0.18 or below. Databricks CLI version 0.18 or below is not supported by Databricks."
So, to answer to you, it is better to go with the version 0.20 although the GA dates are not yet decided and you can always get support if there are any feature requests/issues that you face.