Formidable Form is the more common of the two in our index: it runs on 107 websites against 6 for Riddle. Counts come from real scans of each site's home page, not vendor claims, and cover the sites in TechLookup's index rather than the whole web.
Formidable Forms is a WordPress plugin that enables you to create quizzes, surveys, calculators, timesheets, multi-page application forms.
Riddle is a platform enabling creation of lists, polls, and quizzes.
Within the surveys sites we have indexed, Formidable Form holds 10% and Riddle 0.6%.
Want to know which one a particular site uses? Look it up and see its whole stack, or compare two sites side by side.