You will benefit from a nice high ISO performance gain on the 1DIII, probably worth it for you. Ergonomics are better, but not a big deal. I have both cameras and love both of them.
And I forgot to mention the battery on the 1DIII is just ridiculous...
Agree with the others. The ISO advantage of the III was definitely worth the upgrade to me for the gyms I shoot in with ambient (f2 1/640 @3200) light.
I moved from IIN to III and didn't look back. The IIN is an incredible body. Very capable. But find the III to be better in just about every way. At least 1 stop better ISO performance, better menu/interface system, better LCD, etc. Increase in battery life is very noticeable. I get 3x the number of shots with the smaller and lighter batteries.