Then you've probably found what you're looking for. I've never used the 28-135 lens so I can't be sure the 24-105 is better, but I simply can't believe that a zoom lens with that focal length range could be made much better than the 24-105. Sure, it has some shortcomings (mainly barrel distortion), but they are more or less automatically compensated in the RAW-processing.
If you're about to buy one it would - as already pointed out - be a good idea to wait until the 5D3 kits have reached the market. Then there will be both new kit lenses and probably lots of used 24-105 copies for sale.