Since you have the 18-55MM IS, get the 70-200mm f/4 IS. Then when you are ready, replace the 18-55 with a 17-55mm IS. The tiny gap in coverage will not be missed, but you could upgrade the 18-55mm IS with the sigma 17-70 OS and have complete coverage.


I have had all these lenses on my 40d, and the 24-105mm is wonderful on a crop, but you are missing more by not having the 70-200. Than you would by adding a 24-105 which duplicates or even triplicates the ranges you already have.


In any event, you will certainly love either lens.