I am really sorry, it has been a while since I am back on Math.SE, and since when when does MathOverflow become one of SE family? Won't Math.SE and MathOverflow overlap each other?
The sites (oops, they are no longer sites but communities) are quite different in their focus and thus they complement each other instead of being competitors. Occasionally, people ask:
If I have a maths question, on which site should I ask: MathOverflow or Mathematics.SE?
to which there is a simple answer:
If you have to ask, math.SE