Nash is not a good individual defender, but it's not like he doesn't try. He's actually a decent team defender, but people tend to ignore that point. Many point guards get beat off the dribble, including Chris Paul. The difference is that Chris Paul gets a pass because he has guys like Okafor behind him to cover. Nash has... Channing Frye.
Amare CAN be a decent team defender, but he doesn't try. A couple blocks does not a good defender make. His individual defense is pretty hopeless.
And all this postulating about Amare playing Center. It's all a crock. He's 6'11" 250lbs. There aren't a lot of players bigger than that in the NBA, period, regardless of if they are a center. Even now, there are what--5 players taller than 7' that Amare would try to defend? If you give Amare a pass because he apparently always plays against guys a lot bigger than him, then what about Nash? It's not like he's 25 years old and 6'5".