João Sousa These books are very different one from the other. I would put "Solaris" in the "psychedelic science fiction" book shelf, and "Roadside Picnic" in the "dark science fiction" one.
Chiles probly too late for an answer, but anyway: contrary to what others have said, i would say ROADSIDE PICNIC is complementary to SOLARIS: both examine the effects of something completely unknowable (epitome of 'alien') - that also just happens to somehow provide for the subconscious/subterranean desires/needs of humans - on the psychology of said humans; crucially though, there is an interesting contrast in tone & POV: while SOLARIS closely hews to the perspective of an intellectual (a typically SFnal 'elite' POV), ROADSIDE takes a 'roadside' boots-on-the-ground/working class POV, w/ the effect of the latter being tonally noir; SOLARIS, in contrast, tonally resides firmly in the upper echelons of 'philosophical' SF (though ROADSIDE is no less philosophical - just not tonally, vibing 'pulpy' more than 'brainy')
Steven Have you read Sphere by Michael Crichton? I had some of the same feelings reading that book as I did with Roadside Picnic.
Shtwaan Both of them are as 'psychedelic' or 'dark' or 'brainy' as the other one. More importantly, they use science-fiction tropes to explore socio-psychological issues, in a way that elevates them from the sci-fi ghetto into the realm of real literature. They are very well written texts that stand the critical gaze. It's not by chance that Tarkovsky used them both as source material for two of his most important films.