Good read:
A lot of people just kind of figure that defensive tackles are all the same. When they watch the Redskins they know there are two of them down there in the middle of the muckity-muck, and that's about it.
But in reality the duties are different in the Skins system. The Redskins lack a real heavyweight, Sam Adams-type of two-gap nose tackle. They haven't had a guy like that for a long time. Instead, their linemen are generally a little more limber and little more versatile, but there are definite differences between the two tackle spots.
LINK:
D Tackles: Nose vs. Three Technique - Redskins Insider