The ng-metadata approach is done with Angular 1.x dependencies and a few small helper libraries. ng-metadata allows developers to use Angular 2 style TypeScript (annotations/decorators) without Angular 2. Unfortunately templates are still mostly in Angular 1.x style.
Once an application is converted to ng-metadata style it is very close to Angular 2, but still requires refactoring. In most cases, ng-metadata is not as efficient as ng-upgrade with respect to refactoring time. The payload of an ng-metadata application is smaller, and porting to ng-metadata can be done in an even more ad-hoc fashion than with ng-upgrade.
The general flow of using ng-metadata with an application is:
Install ng-metadata dependencies
Bootstrap root component
Upgrade components strategically
Refactor the codebase to Angular 2
ng-metadata is favored over the deprecated ng-forward.