Tokina 12-24 mm or Sigma 10-20 mm?

I have D5100 and planning for wide lens. Is AF factor to be considered? Then Sigma is the only choice after nikkor which is above my budget. The Tamron didn't impress me. Please suggest the current market price. Would appreciate if suggested polarizer of B+W or hoya of 77mm.

Added (1). Does AF ia a big factor with wide lens. If not I would certainly go for tokina else Sigma would be my alternative plus that 2mm would be a great bonus.
Please suggest.

Of all the generic lenses being sold, the Tokina is the best generic lens

Polarizing filters are NOT effective when shooting with a wide angle lens

The best ultra-wide angle in the world is Nikon 14-24mm, but because of it's high price, people tend to prefer Tokina 11-16mm.

I'd consider the Tokina 11-16mm/2.8 as well. The additional millimeter at the wide end over the 12-24 and the one stop faster maximum aperture are nice to have. The zoom range is very limited though, but on the other hand it has the dirty little secret that it also works on full frame at 15-16mm.

AF is not terribly important. I use mine manually set to the hyperfocal distance most of the time anyway.

If you plan a lot of handheld low light work (e.g.indoor architecture photography), then the Tokina 11-16/2.8 is the way to go. If you don't expect much low light or a tripod will be available, then i'd go for the Sigma 10-20mm. The ability to autofocus still is nice to have.

The Sigma comes in two versions, a constant f/3.5 one and a f/4-5.6 one. Surprisingly, the latter is both cheaper and optically superior.
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Edit:
@Sprinter: You're right about the 14-24mm… But that's a full frame lens, so it's kind of apples and oranges. On FX it's totally awesome without a shadow of a doubt, but on DX, not just because of the price but also the focal length, i'd definitely prefer any of the other aforementioned lenses. 14mm isn't that wide after all.

AF is more of a preference. I don't miss using it on manual lenses. Tokina is Pentax glass, that's why it's great.

While the Tokina 11-16 won't autofocus on your camera, it is really not a huge issue.

At those extreme angles, almost everything is in focus anyway.

For instance, at 11mm and f/2.8 (the shallowest depth of field), with your camera and if you set your focus point set for 10ft, everything from 4ft to infinity will be in focus.

While you would have to manually focus this lens with your camera occasionally, it would be minimal. And you would have a nice lens should you ever upgrade to an advanced camera.

Of course, this focus generosity is only true with super wide angle lenses. A non-autofocusing telephoto lens would almost certainly drive you bonkers (but that is how we did it in the old days).

I tried a Sigma 10-20mm and found it will not work with the AF function of my Nikon D40X. I do not like manual focus and I'll get a Nikon 12 to 24mm soon. I wish Nikon make a 10 to 20mm, but a review said that the 10 to 12 difference may be a reason for the superior quality of Nikon among the wide boys (What Digital Camera Techniques).In US, Nikon 12-24mm costs slightly over USD 1, 200.