Definitely worth it to read both. For one thing, the Memoirs are a great read- very much give you the impression of the man- but pretty much stop at the end of the Civil War, and Grant was to live for decades longer and had two terms as President- and it's over his time as President that there has always been the most controversy. There is a good reason why he limited his remarks post-war to a couple of pages of general observations at the end of his book: he knew most people wanted to read about his war career, and it was for that career that he most wanted to set the record straight ( for example, correcting the Southern propaganda that he, not Lee, delayed the collection of the wounded after the Battle of Cold Harbor). His record as president was far more ambiguous.

Chernow is not a bad historian, but he does tend to like his subjects, find things that are admirable about them and give them the benefit of the doubt. I think Grant is indeed likeable, but William McFeeley's biography of Grant is still the best. There's a very good case for Grant simply being overmatched when elected President, starting with good intentions and but not rising to the occasion when faced with corruption within his administration and resistance to Reconstruction and Native rights. McFeeley does a better job of assessing that than Chernow.