I had in front of the QC 15. Here the points which led me to buy the QC 25. I got my mind you still QC15 250 - otherwise I think the improvements are too low. 1) Smaller Box 2) Design - they are closer to the head 3) NC is a little better, but not essential 4) Sound is equally well 5) The cable is thicker and slightly shorter, which makes it a little more comfortable in daily life.
The only "gotcha" is that the QC15 maybe are a little more comfortable. The QC25 are indeed more comfortable than any other headphones I know, but the QC15 in this regard are a touch better.
Conclusion: The QC 15 and QC 25 give not much, the biggest improvements are in my view an optical nature