There are some key differences. It does not seem that you can limit the transfers to your contacts, but you can specify "Preferred Visibility" for users to see you, even when you do not use Quick Share. Recipients of the quick sign-in screen suggest that this could be available on Chromebooks, iOS devices, and even Wear OS smartwaters, although we would not have all those available, provided the feature is made public . A version for iOS would require a special application.
Not sure when Fast Share will be launched, although it is a function of Google Play Services and probably would not require Android Q. Google may not expect much, the company has warned that it disagrees with the transfer of Beam based Android In NFC to Q. As such, Fast Share could be the only way to quickly send files to local users without resorting to applications (such as Google files) that your friends do not necessarily have.