Ruby & Rails or Python & Django?

Which one would you use to prototype a web app? I’m leaning towards Python & Django, since Rails seems to have some performance bottlenecks. I know Twitter was developed on Rails, but they also said they wouldn’t recommend it. Pownce was developed on Django and they rave about the ease.

Any suggestions? Mostly, I’m doing this to learn but I’d like to make something that could actually be used at my current job.