While this is not FD specific, I have to pitch in with many prior to me:
The 28mm has a wide angle of view. The 24mm (for some reason, its only 4mm different!) has that "WHOA! That IS wide, now!" effect when I look through it. I know the 28mm f2.8 in the FD mount is one of the best bangs for the buck in WA glass. I don't know how the 24mm compares. But, I DO know that the 20mm f2.8 (both SSC and NFD) is a real gem! Its on my wishlist for sure! The reason I mention that is that in your shoes I would get the excellent 28mm and for that really wide, while still optically excellent look, the 20mm. Have a look at
this site - aside from being one of my favourite photo related sites on the web, there is a whole slew of photos taken with the 20mm. I think you will love it.
Another thing to consider are the Krion WA lenses. Real hidden gems (as well as the Vivitars made by Kiron). This may be a way to go if you want an f2 lens that is by all accounts as good as anything Canon made but can still be had for less than the faster f2 wide angles from Canon.
Best of luck - just don't end up buying them all

Or then again, why the hell not!
Peter.