IMHO if you can, newer is better generally, especially in digital even if the difference is slight, its a nice bit confidence to having knowing your a shooting with equipment which won't detract from image quality, so that you can just focus on making the image instead of the technical side when you are out shooting, and leave the technical side for when your in front of the computer on LuLa

And thats more my commentary on new equipment then your specific question. I recently used the 1.4 II with a 300mm f/4 (canon obviously) on a trip, and it seemed to work nicely with that though, can't imagine it can get much better, but still can't hurt right? unless you find the III is worse on whatever lens / application you are using it for