If you take a serious look at Symbian Belle and Meego Harmttan UI you realise they not that different...

Both is using full Qt framework. My guess is also that qt-components for Harmattan and Symbian will merge in the near future.

Differences is with N9 the geeks can tweek a bit more than Symbian in lowerlevel stuff :-)

But the facts that both OS:es now have full support for accessing GPS/Acc/NFC/phone etc... directly from QtMobility API makes it even more easy to port apps between the platforms :-)

This is also why I think N9 WILL be supported longer, both from Nokia and appdevs, longer than case of N900. Many apps may very well work on both platform without massive changes of the code like it was with N900.