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i have both and i would have to say get the sb800. i shoot with my d100 and d70 with my 80-200mm at 200mm at 50 feet away and it lights up the subject perfectly. spend the extra money and get the 800. the 800 also works with the nikon sd-8a battery pack. the 600 doesn't. its a very nice setup.
[Thank Butchwilson75 to be prepared to help me about speedlight and digital cameras. Actually, this matter is, for me, only a curiosity. I've not a D70 and I don't work with one. I wont to make some familiar for with am old F4 and a SL 50DX. I don't see any reason to have problems, but I've. Do you give me some help? Many thanks]
I use the SB-800 on a D100. It works perfectly well. Before I have bought it, I was also looking at the SB-600 and a Sigma flash for Nikon, but the 800 looked far better to me. Also from point of manufacturing quality and the display.
And its stroboscopic mode is great, if you want to try it.