What is the difference between Lactuca serriola and Lactuca virosa?
L. serriola has vertically twisting "compass" leaves and paler seeds; L. virosa grows taller with a purple-flushed stem and produces more of the sedative latex.
The short answer
Lactuca serriola (prickly lettuce) is the more widespread species, identifiable by prickles along the leaf midrib and its habit of twisting leaves into a vertical, roughly north-south "compass" orientation; it's also the confirmed direct wild ancestor of cultivated garden lettuce. Lactuca virosa (bitter lettuce) is less widespread but grows taller, with a purple-flushed stem and bluish-green clasping leaves, and yields meaningfully more of the latex historically dried into lactucarium.