What would you use for a brand new cross platform GUI app, CPython or IronPython ?
What about
 - license / freedom
 - development
    - - doc
    - - editors
    - - tools
 - libraries
 - performances
 - portability  
What can you do best with one or the other ?
 - networking
 - database
 - GUI
 - system
 - multi threading / processing
I can only think of about one "cross platform" GUI app that's remotely tolerable (firefox), and people are complaining wildly about it everywhere I look.
If you want to do cross platform, build a nice, solid model that can do the work you need done and build platform-specific GUIs that use it.
I don't know how tolerable wxpython or pyqt are on Windows and Linux, but the further you get from plain cocoa on OS X, the harder it gets to build and the less pleasant it gets to use.
CPython or IronPython?