I think they are very different lenses that only share a similar focal length.
I have the original 24 2.8 (so not exactly the same as OP's Q) and the 24 1.4 II.
I bought the 24L because I recently started to get into shooting landscape photography this summer. On my first morning trip to a rocky beach shore I found 2 reasons for wanting to buy the L:
1. I dropped my 24mm f/2.8 on the rocks (it was still dark out- lens was fine)
2. Fog rolled in and all my gear got "damp"
the 24 1.4 can also do this:Rainy Day
by Philip DiResta
, on Flickr
Awesome close-ups with nice background blur.
The 24mm 2.8IS has, IS. I'm sure that is good for run-and-gun video to help with jittery footage. Or if for some reason you are into landscape photography but not tripods? Which is weird.
Otherwise, I'd just get the original 24mm f/2.8- it's MUCH cheaper and super sharp. It doesn't have USM, but not REALLY an issue. It still is capable of shooting super sharp images right to the edges on FF if needed. and it is small and light and, um, vintage!