I am hazy on the details but my instructor briefly told us that the 1979 edition had better problems and had topics for the advanced undergraduate. The major thing for us as students was that the 1979 was much shorter to use and concise to the point of being a good handbook.

(We do Theory of Computation/Formal Language and Automata Theory as a dual sem course meant to be both a junior and a senior class and thus the 1st edition suits the course better.)

Googling gives https://cs.stackexchange.com/questions/149072/hopcroft-ullma... (see the question for the links)