XML Doesn't necessarily make it easier. Maintaining and changing things down the road is better. wxPython doesn't have XML examples for every widget and it does make trying new things out more involved. wxPython isn't a pure MVC design. Hopefully a more expert developer will discuss this.
I would suggest looking at the most popular tookits and making an example screen with all the widgets you'll need in each and then testing it in OSX/Windows 7. Shouldn't take you too much time. Screen shots should filter the toolkits down to a reasonable number.
Another approach is to separate your presentation layer completely so that you can use different toolkits on each platform. With two platforms you'll only have to learn the basics of two libraries. If you move to Linux later this would allow you to use QT/GTK+ as well.
I would suggest looking at the most popular tookits and making an example screen with all the widgets you'll need in each and then testing it in OSX/Windows 7. Shouldn't take you too much time. Screen shots should filter the toolkits down to a reasonable number.
Another approach is to separate your presentation layer completely so that you can use different toolkits on each platform. With two platforms you'll only have to learn the basics of two libraries. If you move to Linux later this would allow you to use QT/GTK+ as well.