If you are only tracing American ancestors then it makes no sense to have the World subscription - just take out the US subscription. Of course, if you get far enough back you may need to expand to the world subscription.
But before you take out any subscription, do the 14 day free trial. That way you can get a feel for the site and decide if you want to spend your money there. If you decide no, just make sure you cancel within the 14 days.
I also recommend checking if your local library subscribes to Ancestry Library Edition (most do). That may be sufficient for you if you have easy access to the library.