Duncan is top 10 but not top 6. Jordan, Kareem, Lebron, Magic, Wilt, Russell, Bird.
Garnett is at the tail end of the top 20-25 with Dirk, Barkley, and Malone. You can basically have them in any order, I have Dirk slightly ahead of Garnett.
Dunno why Bird slipped my mind. That said, you could make a good case for Duncan over him based on a number of metrics:
Advanced Stats:
Win Shares
Duncan - 206.4
Bird - 145.8
PER
Duncan - 24.2
Bird - 23.5
VORP
Duncan - 89.3
Bird - 79.7
Defense is where Duncan really set himself apart. Bird was a good defender, Duncan was elite. Leaving aside advanced stats:
All NBA Defensive Team
Duncan - 15 total; 8 First Team (tied for 2nd all time)
Bird - 3 total; 0 First team
Duncan named to both a All-NBA team
and an NBA All-Defensive teams for
13 straight seasons, by far the longest such streak in NBA history. Next closest was nine.
All NBA
Duncan - 10 All-NBA First Team (third all time); 3 Second team
Bird - 9 All-NBA First teams; 1 second team
Titles
Duncan - 5 (3 Finals MVP)
Bird - 3 (1 Finals MVP)
Longevity/Durability:
Duncan - 19 Seasons; 1392 games; 251 playoffs
Bird - 13 Seasons; 897 games; 164 playoffs
Add the fact that Duncan was the best ever at his position.
I wouldn't begrudge anybody to choose either way. If I'm starting a team, I'm taking Duncan over Bird myself.