Haitch is correct (as he usually is). But unless you want the whole Milky way in one go the 24 should be good.
The problem with the 14L is not just the cost but the utility. Even on an APS-C sensor, its going to be pretty wide.
If you can rent one from Teds or Michaels to try that might be a good path to see if you actually have a use for it other than widefield AP.
I believe Sigma offers a range of decent wide-angles in Canon mounts at about 1/2-2/3 the price, so thats another option.
For some reviews, check out:
www.fredmiranda.com or
www.photozone.de .
The latter is an excellent site as it reviews owner's donated lenses rather than cherry-picked examples. As a result, its also free from "I bought one" bias.
Happy snaps!
PJH