Do NOT use the 24L II for astro. It is a complete waste of time. You will be frustrated, like others before you.
It has the most appalling coma (distorted stars in corners). This only shows up in astro. See: http://www.dpreview.com/forums/post/50949062
- there has been similar discussion in this Forum too, but that thread was the best I found.
I do not know how the 2.8 performs in this regard, but typically Canon seems to not have low coma in their priorities. I do have the 24L II and I have spoiled astro shots thanks to the coma - I made the same stick-with-Canon mistake as you.
I now have a set of the Koreans dedicated for astro work. The Korean lenses are exceptional for astro. That they are so inexpensive is an amazing bonus. Manual focus is all you want at night. Live view to focus precisely. They are even sharp at full aperture.
Buy your 2.8 if you need AF in daylight, this may be reasonable for your daytime purposes. But use the rest of your 24L savings and get yourself one of the Korean brands (Samyang/Bower/Rokinon) for night use. Seriously, they are just fantastic at night -- plus, it's at night. No one can see the brand you are using and we won't tell on you. :-)