With regards to the planets themselves, Solaris is covered in an ocean (spoiler rot13: na bprna bs try gung vf erirnyrq gb or n fvatyr, cynarg-rapbzcnffvat ragv

With regards to the planets themselves, Solaris is covered in an ocean (spoiler rot13: na bprna bs try gung vf erirnyrq gb or n fvatyr, cynarg-rapbzcnffvat ragvgl), whereas Tatooine is a desert planet. The systems are different in that Tatooine (as far as I remember) orbits around a pair of stars. Solaris is orbiting between the two stars in an orbit that should be highly unstable but for reasons unknown the planet appears to be stabilizing itself.