|Descendants - Top-down|
At a family reunion, the Top-down format can be spread horizontally across a wall.
The Left-to-right format can be broken up into multiple panels: create a separate chart for each son and daughter of the patriarch or original ancestor, and display them side-by-side.
Left-to-right uses half the paper: more economical, faster to assemble when tiling multiple pages. Also cheaper at the print shop when you're doing poster-size!