Dirk is greater than McGrady.
Dirk is better simply because he has more range and a more accurate shot than "McGravy".
This would not have been true when T-back was in Orlando, when he would attack the basket and be w/out mercy towards his opponents. Yet now that he is in Houston, he has become a jumpshooting guard. In the nationally televised games that I have watched on TNT or NBA TV, McGrady has not shown flashes or even hints of the the player he used to be. Instead of driving to the basket (for fear I guess of his ailing back....boo hoo) he settles for simple jumpshots and 3 pointers. He went from a multi-faceted player to a predictable all-star. As he rushes to the basket, instead of finishing the play he kicks out the ball to another player. Assist! , c'mon your not Steve Nash here. Your Tracy "freakin" McGrady! Mr. 62 pts./ Sir 13 pts. in 33 sec! I mean he's scoring well below last season's average and hasn't shown leadership the same way Yao Ming has.
On the other side of the spectrum, Dirk has shown he can lead a team to victory. Since last year's playoffs he has shown something McGrady's been missing , GUTS. Dirk takes jumpshots over opponents w/out remorse. He drives to the basket like the all-star he is. Dirk doesn't settle for racking up assist but instead helps his team by actually putting points on the board. His range is uncanny compared to McGrady. Dirk's shot is much more fluid and accurate, unlike McGrady's which is w/out arc and w/ too much power. Nowitzki has proved time and time again he has what it takes to lead a team deep into the playoffs. Remember last years finish against the Grizzlies. Dirk sank that game winning shot, not Terry, not Howard, but Dirk. Dirk was last year's 3 point shoot-out winner, and will also be this year.
In conclusion, I would have said that McGrady was better than Dirk a few years back. But now there is undeniable evidence that Dirk Nowitzki is light years ahead of McGrady.