In honor of Darwin's 200th birthday, some research by psychologist Laura Novick on the presentation of evolutionary trees ("cladograms"):
Her research shows that students are much better at understanding the diagram on the left than the one on the right. She calls the one on the left a "tree" and the one on the right a "ladder," which confuses me a bit: the one on the right looks more like tree branches to me.
The lines represent evolutionary lineages. The branch points are speciation events.
The one on the left includes horizontal lines, where as the one on the right has fewer lines. I would use the figure that most accurately depicts the evolutionary history of a group of organisms.
Perhaps students like the one on the left because it can be interpreted as having a dramatic shift and divergence during speciation, which might fit with their ideas about evolution. Plants often make big leaps like that but other organisms not so much.