FoldingText, TaskPaper, and WriteRoom are all based on the idea of using text based user interfaces to make productivity apps. The difference is in focus and intended use:
WriteRoom — Distraction free writing
TaskPaper — Simple list making
FoldingText — Plain text productivity
WriteRoom and TaskPaper are focused on solving particular problems. FoldingText is a more open ended app intended to be used in many different ways. If you just want to write with minimal distraction WriteRoom is best. If you have a todo list to maintain then TaskPaper is best. If you want to mix todo lists into other notes using Markdown syntax then FoldingText is best.