Whether the OP is just a troll and regardless of the initial agenda, I think the topic is actually a worthwhile discussion.
A few things I would comment before saying who I think is the better defender. Dennis has a reputation as a rebounding stat-padder who would compromise his defense in order to rack up rebounds. I don't think that was the case for most of his career. Perhaps in San Antonio (admittedly I didn't watch many of his games when he played for the Spurs) or in his last few seasons with the Bulls, which may very well be the case. But in his prime with the Pistons, he wasn't trying to stat-pad for more rebounds, at least not most of the time from what I saw. His rebounding was invaluable to closing out defensive possessions.
As far as defensive rating goes, which Duncan clearly has the edge, it's a little skewed. Dennis Rodman was often put on the best offensive player regardless of position on the opposing team. He generally had a tougher job throughout the game than Duncan would have. Not that Duncan would never take the toughest big man match-up, but we all know the drill with Pop and how he would keep Duncan off the better scoring big man to keep him out of foul trouble so Duncan could then switch onto that scoring big late in games. So while Duncan may have that duty for 10-15 MPG, Rodman was tested defensively for 30 MPG on most nights. Defensive Rating doesn't take that into consideration.
A perfect example of what I'm talking about are the 2003-04 Spurs where Bowen had what is regarded his best defensive season. That season, Bowen had a Defensive Rating of 97. But look at his teammates Manu Ginobili and Hedo Turkoglu. Ginobili's DR was 93 and Hedo's DR was 94 that season. So were Ginobili and Hedo better defenders than Bowen that season? Or was it rather the fact that Bowen had the task of guarding the best perimeter player on opposing teams? Just something to think about when you compare Defensive Ratings between Rodman and Duncan.
I think Rodman was certainly the most versatile defender. Similar to guys like KG or Kenyon Martin or Shawn Marion, Rodman was given different defensive duties, sometimes a really tough PG match-up like Magic all the way to a young Shaquille O'Neal and every position in between. Duncan guards centers and power forwards. That's it.
Now all that said, you might think I'd give the edge to Rodman. No. The better defender is/was Tim Duncan. Tim Duncan's value to the Spurs defense goes beyond statistics. His presence in the paint deters and alters scoring opportunities. Especially in a Pop schemed defense, Duncan anchors the entire defense. Rodman was entrusted on any given night to stop one player. Duncan (and this goes for any great defensive big man) is entrusted to anchor the entire defense and stop any player on the opposing team in the paint, whether by blocking shots, taking up space, communicating to teammates, closing the lane, and closing out defensive possessions with securing rebounds. His value to a defense had greater impact than Dennis'. Dennis wasn't an elite low post defender (although still very good) or a great shotblocker. He was great at taking charges, switching on picks, and just being a pest in general. But his defensive value beyond rebounding generally involved stopping one player. Duncan, again like most great defensive big man, had the responsibility of anchoring the entire defense and stopping the entire opposing team when they were in the paint or around the rim. To me, I value that more.
It's a good debate though, and I think it's close than most Spurs fans would probably realize or admit. But Duncan is the better defender. Compare Duncan to guys like Hakeem or Ben Wallace, it's not contest. Duncan would lose that comparison. But against Rodman, I think Duncan was the better defender.