I think that in-class differentiation can be comparable to single subject acceleration, particularly in early elementary. It all depends on the school, on the class, on the teacher, and on your DS. However, it is far more difficult to effectuate. There is really no single right answer and the better approach may change over time. I would take it year by year.
I have a 7th grader who did not subject accelerate until 2nd grade although he knew intuitively how to multiply/divide in his head ($10 roll divided by 25 cents equals 40 quarters) by K and was very disappointed with the math curriculum. One of my main reasons for waiting was to get him to the point where he was mature enough to travel alone within the building and skilled enough to keep up with the writing requirements inherent in our math curriculum and not be treated as a second-class citizen. I actually requested and would have preferred in-class differentiation but it wasn't an option.
