These two sci-fi movies are very comparable in film history. Tarkovsky's Solaris (1972) is often seen as a response to Kubrick's 2001: A Space Odyssey (1968).
2001: A Space Odyssey is full of symbolic meanings. It is a detached view, a presentation without a visible presenter, a visual symphony, about human evolution in relation to technology from prehistory to the future. It offers a place to reflect upon the interrelationship between man and his tool through time. The special effects experimented in the movie was truly pioneering.
For Tarkovsky, cinema's best strength is to observe life, as a mirror for us to look into ourselves. Even as a sci-fi movie, Solaris tries to jump out of the genre by minimizing the special effects while focusing on characters' phychological world. Unlike typical outer-space-themed movies, the mysterious planet solaris interacts with human by duplicating the significant persons in one's life, triggering a dialogue within one's own phyche. Although the story happens in outer-space, it is nevertheless a exploration of our inner-space.
If 2001: A Space Odyssey is an objective, loud, high-speed and outward argument, Solaris is a subjective, quiet, slow-paced and inward journey.