Amusingly, they are both art movements that defined an era. Art Nouveau was strong post-Victorian era (approximately 1890 – 1910), when art and design started moving out of the realistic and into a bit more stylized representations of reality. It was meant to pull the audience in, to make them feel Nature and all Her wonders, not just look at them. And it was a movement not just limited to art, but to architecture and other forms of physical design as well.
As the world moved more towards war, the flowing shapes of the Art Nouveau period became more geometric and moved into the Art Deco period. And, that also meant that art and design were able to open up to new options for how they viewed the world.
If you actually look at the gradual progression through the art movements of the last 200 years, you can actually see how gradually art and design has become far more than just simply showing the world what is there.
Sadly, however, I think that the Modernist movement after Dada and the Surrealists went too far into deconstructing the world. The art made in the latter half of the last century, and most of the art of this century, has moved so far away from the world as to be unintelligible to the average person. And I think that is a great loss for art.
But, I have hope. I see what street artists are doing, and what artists outside the “established art movement” are doing, and they don’t look at the average person as inferior. They want to reach the average person, to make them FEEL.
That’s why I love Dali and Mucha so much. To me, their work is about exploring the world around us, whether it is the physical world or the metaphysical world of Idea. And seeing art like the one above makes me think as well as feel.