The correct statement when comparing the political systems of Mexico and the United States regarding presidential terms is:
Mexican presidents cannot be reelected after serving six years, regardless of job performance.
In Mexico, presidents serve a single six-year term (known as a "sexenio") and are not allowed to run for reelection. In contrast, U.S. presidents can serve a maximum of two four-year terms and may be reelected once. Thus, the statement about Mexican presidents’ inability to be reelected after a single term is accurate.