Dealing with Unicode in Python 2.x is a _nightmare_. So if you need to go beyond ASCII, Python 3 is a huge improvement. On the downside, if you want to use tools like App Engine, you're still stuck with 2.7 for now, but hopefully that will change.
I'm sure I hit Unicode problems in Python 3 a few years ago as well, but if current versions are better that would be a sufficient reason for switching.