Do I need a stiff or soft brush?

The surface being painted, the paint and desired finish will determine the stiffness of brushed required.

If you are cutting in or painting around doors and trim with a thicker paint such as oil-bases or water-based gloss then you will want a stiffer brush that gives more control and crisp lines. However, if you are painting doors, furniture and trim and are looking for a smooth finish with less brush strokes then a softer brush like the Purdy Syntox or Wooster Silvertip is recommended.