You wouldn't choose one over the other, but you may want to use both.
 
Montage is a screenwriting app for the Mac.

Contour is a story and structure development tool for Mac and Windows.

From the Contour page, "Contour is a proven story development system that streamlines the process of turning your movie ideas from first glimmer to full outline. Using the Contour story development system created by Emmy Award-nominated Jeffrey Alan Schechter, you will learn how to use the same character-based structure that nearly all the blockbuster movies use to create solid stories that hold together from Fade In to Fade Out."

You may choose to begin in Montage, then once you get the basics into your screenplay, you may start wondering about the overall structure. What are my plot-points, how much time am I spending on those points, etc.? Then when you solidify the structure, you go back to Montage and manifest that developed story, to get you back on track.

All of your products are available online to download and use in trial mode for several days to help you decide if they are right for you. Get them from our downloads page.