If the steel is different as C9 says it is, I'd go for the 600D even though I don't particularly like the way it looks. I tried it out for Gator's passaround and I was impressed at the quality and performance at that price.
I haven't used the 600S or 600D, but have one 'Twin Hocho'. In Japan, Henckels has been selling this line for a while. It has a 3 layer blade with a core of microcarbide steel of HRC63 (SG2?) and the very same handle of the 600 line.
Based on my experience with this baby, some Miyabis and from what I've seen of the 600S & 600D, I'd put it that way:
Although the 600D is marketed as Fusion, the 600S is the real 'fusion' knife, as this basically is a Japanese Gyutoh made of German steel. I haven't handled the 600S, but used their 5000S line for a while, which has a similar concept of 'Japanese blade' with German steel. Actually I liked the blade. It was very thin and held the edge well for a German steel. I wasn't 100% comfortable with the handle. But this issue is sufficiently addressed with the 600S. Thus the 600S IMO makes a good entry level knife for those who want to move from German to Japanese knives, but appreciate ease of sharpening and maintenance of the 'Germans'.
The 600D is more Japanese in a way that it uses Japanese steel. It's more expensive too, but with harder steel, a more flashy look and the Damascus, it'd say it's a better value than the 600S. Sharpening is a bit more of an issue here, but if you don't want to start using whetstones, SLT is selling a tungsten carbide steel (although not really cheap...) for the 600D.