Contact Form 7 is the more common of the two in our index: it runs on 1,148 websites against 52 for Image Placeholders, and 6 sites use both. Counts come from real scans, not vendor claims.
Contact Form 7 is an WordPress plugin which can manage multiple contact forms. The form supports Ajax-powered submitting, CAPTCHA, Akismet spam filtering.
Performance plugin from the WordPress Performance Team, which determines and stores the dominant color for newly uploaded images in the media library, then uses that as a placeholder background during front end image loads.
6 sites run both Contact Form 7 and Image Placeholders. That is 12% of the sites using Image Placeholders, the smaller of the two.
Want to know which one a particular site uses? Look it up and see its whole stack, or compare two sites side by side.