Actually, Pluto has been known to talk, having done so twice in Disney history. The first occasion occurred in 1931 with the Mickey Mouse short film, "The Moose Hunt." In the film, Pluto is heard to say "Kiss me."
Since the early films, Pluto has stopped talking, the result of stabilizing his character by Disney. Instead, he responds in grunts and barks. Goofy, on the other hand, has been speaking since his creation in 1932. While Pluto is essentially Mickey Mouse's pet dog, Goofy is characterized more as Mickey's friend. Because of this Goofy was anthropomorphized, completely dressed in pants, a shirt, a vest and a hat, and given more human emotions and motivations. Pluto simply acts as a dog.
Goofy is so humanized that the name Goofy appears to be only a nickname. In the 1950s, he was known as George Greef or G.G. Greef. In the late 40s, he was known as James Boyd, and in the comics produced during the 2000s his name was given as Goofus D. Dawg. Goofy spent his early existence nearly silent like Pluto because the original voice actor left Disney. During this period, the character appeared in a series of how-to videos which carried a voice-over narrative.