I haven't pointed the Sigma 10-20mm into the sun, so I wouldn't know; I hear it flares somewhat less than the Tokina. One of the guys I shoot with has the 10-20mm (not quite sharp on both sides), so if I see him I can ask for you. I hear that it's a decent performer if one manages to find a good copy, but I haven't seen a hypothetical good copy in person, and neither has Julio/Nikonfreak.
Here's a handheld grab shot from the Tokina 12-24mm f/4 straight out of the D2HS (resized for web):
From what I've seen produced by the Nikkor 12-24mm f/4, the flare performance is close to if not on par with the 17-35mm f/2.8D AF-S, which is the standard for flare performance when clean inside.