Nomura does a lot of other Square games, not just the Final Fantasy series. He acts as other roles, too. For example, he was the director of Kingdom Hearts.
Amano has done art for Seven, Eight, and Ten, but he didn't design the characters. For Seven and Eight, I'm actually glad he did the designs, eventhough Amano is my favorite artist from Japan. Amano's designs just wouldn't have looked right when transformed into 3D. Also, the dark theme of Seven, and the realistic look of Eight fit perfectly with Nomura's designs.
However, I think Amano could have been able to do Ten really well. I look at some of the garments and outfits of the characters in Ten, and I think "wow, I wonder what they would have looked like if Amano drew them?" Don't get me wrong here, I love Nomura's designs for the game, I'm just curious to see what Amano would have done with them.