If you are looking to shoot the Milky Way with a crop sensor you'll want to go as fast & wide as possible. The wider the lens, the longer the exposure can be without generating star trails. The longer exposure in conjunction the fast 2.8 goes a long way in keeping ISO levels down. Dave Morrow's Blog has some excellent information on this including a handy reference chart:
http://500px.com/blog/738/tutorial-photograph-the-night-skyBased on what you've said above, I'd recommend the Tokina 11-16mm f2.8 as a lens worthy of serious consideration for pairing with your T3i. My experience with the Tokina is this:
Strengths:Clarity and sharpness are excellent.
Chromatic Aberration - Never had any significant issues
Produces gorgeous sunstars.
Excellent build quality
Less expensive than name brand alternatives
Can be use on a FF camera at 16mm with very respectable results.
Weaknesses:Generates harsh and ugly lens flare when the sun or other extremely bright light is in the frame (a pity, since the sunstars are so nice)
Creates horrid hot spots when used for near infrared photography, even at widest apertures.
I replaced my Tokina 11-16mm with the Nikon 10-24mm F4 because the hot spots were a deal breaker for me, as I shoot infrared extensively. Other than that and the flare (which is always worse in infrared), the Tokina is a better lens in every other aspect. It's significantly less expensive too. I realize you shoot Canon, I mention this comparison only to highlight that the Tokina holds up (and in many ways outclasses) more expensive name brand glass.
If you are looking to shoot astral bodies, I'd recommend investing in good telescope and tracker with a Canon camera adapter mount instead of a lens.