Our district does virtually no differentiation for math in the classroom (unless there are random teachers who do it). The district wants everyone to get the same lecture and the same worksheet everyday. So math has always been a much bigger problem. With reading, they can simply be given more challenging books, and for K-2, at least, there was ability grouping for reading. I think most classes will have a few kids who are advanced in reading (unless you count kindergarten which may not have ANY readers) and the spread is much bigger. But for math, very few kids will be working 1 or more grades ahead.
In DD's case it was clear she needed to be accelerated in both math and reading so they did a whole grade acceleration. If I had to choose one or the other, in kindergarten I would have chosen reading because the instruction there was so clearly ridiculous and took up so much time. After that I would have gone for math because in first grade most kids are able to read independently, and it seemed less ridiculous. In first grade the kids didn't spend nearly as much time sitting through silly lectures like "A says Ahhhh".
