Brush vs pencil

From Anipedia

In flash animation, there are initially two tools that draw;

One is the Pencil and the other is the Brush.

The Pencil has a defined width and won't become accented with strokes. It has a smoothing and straightening function that can be applied to it after the base stroke. A third function, ink, has it set down the lines exactly as drawn, with no changes applied.

The Brush on the other hand will imitate real brush strokes depending on the smoothing level applied to it. It too has smoothing and straightening functions, along with varying brush sizes and shapes to paint in.

The Pencil is preferred to make the overall design to seem controlled and neatly done, because it can be more easily tuned up afterwards to look closer to the artist's specifications.

And the Brush is less uniform and clean looking due to frequent mistakes in the smoothing algorithm, but an artist can move the edges of the finished fill or merely redraw the stroke.

The Pencil tends to have smaller file size because less code is required to render a line as opposed to a fill.

The Brush tends to have higher file size due to the extra code required to render a fill.

The debate on using Pencils or Brushes to outline figures is one of personal preference.