I see async-await as a way to simplify the syntax of Promises, but you still are dealing with Promises, i.e. with a model that allows you to deal with 1 event at the time.
Observables on the other hand are models for ‘streams’ of events and, based on this, offer a vast set of operators that allow you to transform streams of events into other streams of events, applying functional style coding.
At the end, anyways, it is a matter of taste. I personally think that once you have started getting used to reason about streams of events, i.e. Observables, these represent a nice modelling tool for scenarios in asynchronous non blocking environments.