Which is better: praying behind an imam who shaves his beard in a mosque with many worshippers, or praying in a mosque with a small number of worshippers who follow the Sunnah of the Prophet (peace and blessings of Allah be upon him)?

Insisting on shaving the beard is an act of transgression (fisq). Praying behind a transgressor (fasiq) when there is someone else more worthy is disliked (makruh). The deficiency in the imamate of a transgressor is not rectified by the large number of worshippers behind him. Praying behind an imam who upholds the Sunnah, even if his congregation is small, is better and more preferable than praying with a transgressing imam, even if his congregation is large, because the prayer of a transgressor is deficient and disliked.