One important distinction here is whether we're talking about prokaryotes, i.e. organisms without a complex cell structure, and/or eukaryotes, organisms that have nuclei and other structures. This would significantly affect the comparison.
However, generally speaking, we can say that unicellular and multicellular organisms are alike in that they exhibit all the functions of life, such as a metabolism and reproduction, they contain DNA and RNA, they can exhibit a wide range of lifestyles, and they are essential to almost every ecosystem that we currently know of.
