Wide Angle Prime - Which is Better: Nikon 20 mm f/2.8 or Sigma 20 mm f/1.8 ?

I am considering buying a wide angle prime for my D7000. I already have a the Nikon 35 mm f/1.8 but I want something that has a wider field of view. I am now considering a 20 mm. I see that Sigma makes an f/1.8 while Nikon has f/2.8. From the specs I understand that the Nikon is lighter and has a smaller profile than the Sigma lens, while the Sigma version of course opens up wider to f/1.8.

I plan to use this lens as a general walk around and travel lens.

If anyone had some experience with either of these lenses? I would appreciate hearing your thoughts and experiences using them.