I recently went through the same decision-making process that you are going through now. I started with the K702, but found that the sound signature didn't suit my tastes.
Then I auditioned both the 880s (albeit the 250 ohm version) and the HD 650. To my admittedly untrained ears, I thought that both were great phones with differing sound signatures. As I listened to them (i.e., through a PAV2) I found that both had nice, although not overbearing bass. The 880s had more perceived clarity in the upper range. The 880s had a nice, although somewhat clinical sound (not as much as the 702s, though.)
The 650s didn't have the clarity in the upper range, but, for me, I felt the overall presentation was a bit more cohesive. In the end, I just found the 650s to be a more pleasing (if not more accurate) sound. I felt like with the 880s I'd be standing outside the music analyzing it, while with the 650s, I'd just want to crawl inside the music and let it flow around me. Of course, YMMV.
Now, truthfully, the price difference is quite significant. At the time, (about 2 weeks ago), the 880s could be had on Amazon for just south of $250, while the 650s were $352. I ended up getting them at J&R in NYC for $356 because there was (I believe) a price sticker that should have been removed, because they kept saying the price was $499 and they wouldn't match the Amazon price. I now realize that I got a very good deal because it seems like you can't get the 650s new anywhere for less than $499 anymore.
For the $100 difference, I'd say it was worth it. For a $250 difference, I wouldn't be so sure.
