Desert Gecko Loc: desert southwest, USA

I currently own a Tokina 11-16mm f2.8 ATX Pro and I'm pretty satisfied overall. I like to shoot stars, though, and I'm disappointed by the coma I get wide open toward the edges and in the corners. I'm curious if the Sigma lenses might be any better, or even if I might just have a bad copy of the Tokina.

I realize that the longer focal length would reduce my max exposure for astrophotography, but the gain of a stop & a third makes up for that in light transmission.

Does anyone have experience with either of these Sigmas? And how about the Tokina? I've read a couple threads here wherein it was discussed and other than it being prone to nasty flare, there was overall agreement that it's a fine lens. Anyone else have coma issues with the Tokina?

Screamin Scott Loc: Marshfield Wi, Baltimore Md, now Dallas Ga

Had the Sigma 20mm F18 & returned it... Hated it's wide open performance & stopped down, it wasn't any better than a 20mm F4 Nikkor I own.

MT Shooter Loc: Montana

I have the 20mm f1.8 and was not terribly impressed until I fine tuned the focus, now its a superb lens. Unfortunately I use it very little lately since getting the Tamron 15-30mm F2.8 SP VC lens, its killer sharp and a fantastic performer.
I have used my rental copy of the Sigma 18-35mm f1.8 a few times and it's a great crop sensor lens. Unfortunately I seldom use my 7D200 and then almost always only on my long teles to gain that extra "reach".