This question comes up fairly frequently, so below are links to previous times that these books have been discussed. This list was compiled by /u/Searocksandtrees a year ago, so there are likely more recent discussions missing. I've read both and my specialty is in Latin America, so for what it's worth, my thoughts are that the books are a fairly good overview of native American history, including the presentation of theories that are well attested by historians but rather underrepresented in the general public. Of course, Mann is not an academic historian and as such receives some criticism for his inconsistent application of methodology and under-researching of certain topics (native Andean civilization comes to mind for me). I enjoyed the books though and I think that the accessibility and broad scope are worth the tradeoffs, especially if you're looking for an introduction to the literature and/or only have time to read two books!