New app, mid development. Switch to Qt5?
Am I correct in that QML2 + v8 is very different from Qt 4.8's QML + script?
Is Qt5 in good enough shape to build a commercial application on now (tentative release by end of year) ? Or would we be chasing a lot of bugs still?
Yes and no.
Yes, the backend for QtQuick2 is very different.
No, the QML frontend has just gone through minor changes. See "QML Applications in Qt 5":http://doc-snapshot.qt-project.org/5.0/qtquick-porting-qt5.html.
There are still some issues with Qt 5 and some minor API changes going on, which might or might not affect you. We are using Qt 5 for two projects and both aren't at a relevant scale.
As to the question if there will be a commercial-grade Qt 5 release by the end of the year. Well, maybe, or maybe not. It is at least planned as far as I know.
The "releasing mailing list":http://lists.qt-project.org/pipermail/releasing/ is a good place to start if you want to follow the state of Qt 5.
One major design goal of Qt 5 was source code compatibility with Qt 4. So you might release your application with Qt 4 and port it later on to Qt 5.