Python (with Django) and RoR.
If you had to choose one, which would you pick?
We went with Python + Django. It was an excellent choice for us! We don't have any performance & scaling issues so far (and we have a pretty big site already) and programmer productivity has been nothing short of amazing.
In the end, pick whatever you feel more comfortable with and whatever the market you have company in supports. If it's easy to find RoR folks around where you live, use that... if it's easier to find Python people, maybe you should use Python instead. They're both excellent languages and a huge improvement over Java/C++/C#.