I prefer the QC15 case 'cause it's slimmer and fits better in my brief. You can just about fit the QC25s in them with some wiggling. I borrowed a pair from a friend and walked around with them for a day. It's a welcome change but not groundbreaking or revolutionary. It feels more like the difference between the iPhone 5 and 5S.

In the subway there was a difference, the noise cancellation was noticeably better and the most improved feature over the QC15s. The ear cups did feel lighter on the side of my head.
I'm not an audiophile but I did find the bass tighter and the soundstage a bit wider which was nice (I purchase my music on iTunes).

I didn't feel compelled to upgrade from my QC15s. They're still in good condition and only 3 years old. This is all subjective though, I don't consider myself to be very tech savvy. It's worth popping down to your local Bose store and taking a look (and listen).