Observable Vs Promise. All you need is here.

Brief

An observable does everything that a promise does and more. It can always be switched to a promise with toPromise() method in case a promise is expected.

An observable must be chosen over a promise if

• any feature that are intrinsic to observables and not promises and explained in detail in related question is in demand (notably incomplete observables and observables that receive multiple values)
• API that consumes it expects an observable and doesn't use Observable.from(...)safety structure to unify observables and promises

An observable may be chosen over a promise if the code where it's used uses observables exclusively.

A promise must can be chosen over an observable if API that consumes it expects a promise and doesn't use Observable.from(...) safety structure.

A promise may can be chosen over an observable if

• the code where it's used uses promises exclusively (notably async functions)
• it needs to be immediately subscribed and chained then, because a chain should be broken in observables let observable = ...; observable.subscribe(...); return observable (this also requires multiple subscriptions to be tracked in case an observable is cancellable)

Link

1. https://stackoverflow.com/questions/50269671/when-to-use-promise-over-observable