Surfing the first wave
The WWDC keynote made me curious enough to download and install very first versions of both operating systems, previews only available to registered developers: Although I was able to identify minor issues with StitchBuddy's Quick Look plugin and the stitch simulator on OS X 10.9, these might be caused by the very early state of Apple's preview. I will check again at a later more stable state, and I am confident to resolve any possible problem. iOS 7 is currently only available for the iPhone, so I checked StitchBuddy View, and found it working, yet with an unfamiliar appearance.
This was only a very first glance, both OS are still subjects to change in the next months, but it was fun surfing the first wave and finding my apps working well…
Update (2013/07/12): Apple has fixed the mentioned Quick Look issue with Mavericks 3rd Developer Preview (I filed a bug report), and I figured out how to fix the Stitch Simulator. I'll incorporate this minor change in the next update, and everything will be fine for the next OS X version.