Of course, I'm biased but wouldn't TD offer a bit more post offense than KG - something a bit different from KG who is probably more similar to the way Bosh plays? TD does play more in the post in the playoffs than the regular season.
Also think TD would be better against Dwight/Gasol and Randolph/Gasol - not the rest of the league though - as the trend toward smaller players continue with KG and Bosh now playing center.
Timmy would allow everyone to go back to their natural positions instead of Bosh playing C and Bron playing PF like they have been. Plus Duncan gives them offense from the post and a defensive anchor on the inside.
Duncan would be the center that they would need. Almost the perfect center that they would need because of his versatility. I don't know how he would affect them in the locker room (doubt he would affect it as KG). He can space the floor as well as attack the post. I think he is more versatile on the offensive end at this stage of his career, and he'll give you more rebounds. And he'll put everybody in their natural position. And with Duncan, they would get bigger.
KG provides more toughness, but he is mainly a jump shooter at this stage of his career. While it's not bad because he stretches the center position, but Duncan could do the same thing. His defense isn't the same as it once was, but that could change.