How can I conditionally use either [href] or [routerLink] on an anchor?

The simplest way would be to use *ngIf / else:

<ng-container *ngIf="outside; else internalBlock">
  <a [href]="externalUrl">External</a>
</ng-container>

<ng-template #internalBlock>
  <a [routerLink]="['/route', id]">Internal</a>
</ng-template>

EDIT#1: (Ugly workaround)

Since you don’t want to use *ngIf (I still don’t understand why), you can do this:

Template:

<a href="javascript:void(0)" (click)="handleClick(outside, '/route', id, externalUrl)">Link</a>

Component:

handleClick(outside: boolean, internalUrl: string, internalId: string, externalUrl: string): void {
  if (outside) {
    window.location.href = externalUrl;
    // You can also use Location class of Angular
  } else {
    this.router.navigate([`${internalUrl}/${internalId}`]);
  }
}