When comparing Default Folder X vs Notational Velocity, the Slant community recommends Notational Velocity for most people. In the question“What are the best power user tools for macOS?” Notational Velocity is ranked 11th while Default Folder X is ranked 39th. The most important reason people chose Notational Velocity is:
The interface is clean and unobtrusive.
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Pros
Pro Directory assistant Finder Integration
From the Open / Save file dialog, you can access your recent, favorite, and open folders. Includes a menubar item and can add to the Finder toolbar for quick access.
Pro New features are intuitive
Many of the features provided by Default Folder are quite intuitive and quickly become very useful. When a save dialogue is open, just click on an open Finder window to change the current path to that directory. Integrated Quicklook previews make selecting files in the list view much easier, and Spotlight tags can be added directly from the save dialogue.
Pro Simple and minimalistic
The interface is clean and unobtrusive.
Pro Free
Pro Syncs with SimpleNotes or Dropbox
Cons
Con Feels a bit outdated
The last release was from 2011. Although due to its brilliant simplicity in doing what it's designed for, some might think there hasn't been any need for an update, as long as it still works with the newer OSs.