But I've repeatedly found myself wishing I'd gone with Ruby/Rails (it was basically a coin toss.) because I've ended up in an environment where everyone else is working in Ruby for their pet projects - and it would be great to be able to tap their knowledge.
So, have a look around, what are people you know using? It's probably really useful to choose something that your friends know (if any of them code), or that you know there's a decent support for in your area - but beyond that, it doesn't really matter. And Python's really nice.