Nikon 10-24 mm or Sigma 10 - 20 mm?

The salesman at the camera store felt that the Sigma was better value at $550 vs $860 for the Nikon and he felt that it was the same picture quality. The Sigma is f4.5-5.6 the Nikon 3.5-4.5

It is for landacapes so I don't see speed as a factor. My concern is picture quality.