I believe Get Data will return any data from the layer that matches the where clause you add in this action, where as the Fetch updates returns any data that has been been modified since the last time the flow has run by looking at the change log of the feature layer.
One issue I have come across is it does not seem that the Fetch updates returns newly created features, only edits that have taken place to existing ones. And for Fetch updates there is no way to have it only look for certain updates (like a specific field) it's just any time a feature is updated in any way.
