Both are same and devoted to assess the quality of journals based on number of citations. Impact Factor is generally accepted one while CiteScore is recently introduced by Elsevier's SCOPUS.
impact factor is quality of papers published by a journal with peer review process and other criteria which judged by eminent researchers. cite score is rate of citation of paper and paper from a particular journal depending upon the availability of same to fellow researchers. I think citation score not link necessarily to cite a research from prominent journals only rather the best suited to you and available a point of time. Now days Scopus, googlae scholar, RG and Acadmia etc are available to dispense your article so have more citation.