I'm not adverse to picking up the Rokinon 24 if it is actually good for this type of shot. The problem is I understand it can be soft, and I'm not sure what use I would have for it besides night shots, where the 24L I could see myself using more.Wrong Rokinon lens. Get the 14mm f/2.8, generally considered one of the best astro lenses out there. It's got a hard-stop for infinity, and the f/2.8 is plenty at that focal length. Honest truth is many lenses are not great for night work, as they are designed for other purposes. There's only a small handful of lenses that are ideal, and that Rokinon is the cheapest.
Night photography needs 3 key things
1. Dark skies (way away from cities, you can see here: http://www.jshine.net/astronomy/dark_sky/). No combo of high ISO and f/1.4 will make up for light pollution.
2. High ISO/large aperture (5D3 is a good option)
3. Wide angle (allows you to take slower shutter speeds)
The difference between your 24-105 at 24mm f/4 vs 14mm f/2.8 in night photography is pretty big, and in many cases, will be the difference between an ok night shot, and a great one with the milky way clear.