I think the 16 is dramatically better between f1.4 and f.5. I can't seem to get anything useful out of the 10-24 in this range...
Seriously, the f1.4 and f4 are "miles apart". Each has a certain use and as my old photography teacher was saying: why bother paying for an f1.4 lens if you are only ever going to use it at f8 to shoot your boring wheat fields and beaches?
Personally, I am also undecided here. I have some use for the f1.4, but 16 is a little too wide for my taste. I'd very much prefer having an 18/1.4. On the other hand, I have some use for 10-24, but f4 downwards rather than f1.4. I guess: the 10-24 is a lens on my shopping list, while the 16/f1.4 is having a hard time against the 18/f2.