When to use decorators

Are you sure?

Decorators decorate your class with new set of functionality.

What are your decorators doing? Adding a few rendering specific methods to a class to help with rendering? Perhaps you should consider Presenters. But better: how will it scale, can it be grouped, will it really add the simplification. Be wary of too quick branching off functionality to decorators. Most cases I've seen them were overly architectured, and they didn't bring much value.

One might consider using Concerns or mixins as an alternative. The disadvantage here is that your main object gets more public methods, but I consider it as a feature after having experienced too much potentially reusable functionality grouped arbitrarily away in other presenter / decorator classes.

When to use Form-objects?

When necessary.

It depends. By default I would advise against them; not creating Form objects to receive and validate data that could be validated by the Model directly. Even when you have a few nested attributes that belong to the main model modified, I would advise against Form objects. Keep It Simple.

But… sometimes you have more complex forms that don't fit the database-mirroring ActiveRecord model as nicely.