Check out this thread here before you make a decision.
Of course the 1DMkll is a better camera than the 20D. The question is, are the extra features good value for you? Do you need 8 frames per second and the slightly higher resolution that the 1D can give you? Faster focussing and more accurate focussing are always welcome and I expect the 1DMkll will often be better than even a properly adjusted 20D in this regard, but a properly focussing 20D might suit your purposes well enough.