Differences
Global
Announcements created in this feed will be displayed to users who have access to your Jira or Service Management.
However, it doesn't mean, that all users will receive an announcement - in the further part creating feed you will define Audience more precisely.
Announcements will be displayed immediately after logging into the system before the user takes any action.
Project
Announcements created in Project Feeds will only appear when a user has access to a specific project.
This means that users that don't have view privileges or participate in a specific project won't see the announcements created in this feed.
However, it doesn't mean, that all users who have access to that project will see announcements - in the further part of the creation of the feed you can define Audience more precisely.