Opinions - Which might be better?

I am sure the answer to this question is going to be the ubiquitous "it depends" but here goes...

I am looking to shoot some astro stuff and the two lenses I am considering are the Sigma 30mm 1.4 and the Canon 15-85 3.5-5.6. The question is this:

Would it be better to shoot the 30mm wide open or even stopped down a bit (2.8 maybe?) or to shoot at 15mm at 3.5? I am sure it makes a difference but since both of these are crop lenses I'll tell you that I am shooting a T3i.

Advise/guidance would be helpful..

TIA..

The faster f stop is always a great advantage in astro photo's. If you're not using a tracking device to follow the stars then even more so. The 30mm on the crop body is a 35mm equivalent of 48mm, which isn't very wide, so this will limit you even more in the exposure time, again, if you're not tracking. Meaning the wider aperture will be a bigger advantage.

You should be able to get 30 second exposures no problems out of that 15mm. I'd go for that. 30mm is just too restricting if you want to capture the sky.

Thanks to you both...