Jackal's got it right. Neither of them is anything more than a marginal HOFer at best, but you can certainly make a better case for Puckett, who played a more demanding defensive position, still hit as well or better than Mattingly, and did not have the decline at the end of his career as Mattingly did.
At best, you can argue that Mattingly had one season where he was the best player in the game (1986--even in 1985, Brett was much better). Mattingly was only a dominant hitter for less than half of the 1980s, 3-4 seasons (1984-1987). From 1990 onward he was a below average first baseman at the bat, and that's about 40 percent of his career.
http://www.baseballprospectus.com/artic ... icleid=856
Guys who are dominant for 3-4 seasons, average for 2 seasons, and mediocre to poor for 6 seasons don't belong in the Hall. The ONLY reason he gets considered is because he's a Yankee.