In a 4-3, the nose tackle isn't two-gapping. He's asked to shoot through the gap and penetrate the backfield. As a defensive tackle gets older, he loses the ability to run. In a 3-4, the nose tackle is more of a wrestler. Strength is more important than leg speed for a two-gapping nose tackle.
This is the reason that Cody will not be a Saint next year. I really don't know how else to explain this to you.
Also, Saints opponents are always playing from behind because our offense is so good. Therefore, we need DT's that can rush the passer, which Cody cannot do. It's a new NFL Canton. Stopping the run and running the ball are the old school way of winning. Rushing the passer, passing the ball, and defending the pass are the new school way. The pick will be DE, DT with rush capabilities or Athletic OLB that can defend the pass. Think Sean Witherspoon.