I would look at the 1Ds3 as the pro version of the 5D MarkII. In my opinion it's the best DSLR that Canon ever made. I'd have one if I had the extra cash sitting around to get a mint body. But I'm sure a lot of people would disagree with that. I look at cameras still as replacement for film. Anything over, say, ISO 800 is more or less irrelevant for me. Nice to have at times, but not a selling point. I recently brought along my 5DII and a rented 5DIII and did not see enough of a difference in the results to justify the upgrade. Yes, the AF is faster and better, but also more cumbersome to fully take advantage of. So I ended up getting about the same keeper rate.
6D may be interesting for street photography because of its size. For studio or anything else where you work with flash or strobes I'd want something with a better x-sync speed though.
I personally would get the 1Ds3 for studio and take the 5D for street.