On one hand, Winslow Homer was a self-taught artist that became proficient at oil-painting as well as water color, and he made vast advancements in painting technique that are only visible thanks to today's x-ray technology. His work applies "fugitive pigments" and other advanced watercolor techniques that are difficult for anyone to master. It is safe to say the Homer was a painter ahead of his time, and that he is a master painter important to the American artists portfolio.
On the other, Edward Hopper captured the nightlife of New England, the casual everyday American just doing what they do, and the atmosphere of the community he was surrounded by.
The question then becomes; What makes an artist American? Is it their self-taught skill or their subject/content matter?
Personally, I see Edward Hopper as more of the "quintessential American artist" because he was depicting the natural scenes of America and the people of this country. Homer's techniques make him a very important American art figure, but his paintings do not give an overarching impression of being "American."