I would go with Django over web2py. If you're looking for more of a microframework, I would look at Flask.

Having built enterprise applications in Rails and Django, I would recommend Django over Rails 99.999% of the time. Django is a very homogeneous framework; it's easy to use, learn, deploy, maintain and has built-in ways to do just about anything you'd need to do in a web application.

Rails is more like a piece of flypaper, that you trow a bunch of stuff (gems) at and pray they stick. You can't even upload a file in Rails without using a plugin.

I guess, the first question is Python vs Ruby. I use either and don't have a strong view to be honest unless you do so number crunching in which case Python offers more in terms of libraries. As to which webframework to use, RoR is indeed a bit like a fat man but so is Django. If you're new to web dev, I'd say go with something lightweight, ie. web2py or even flask in the Python world or Sinatra in the Ruby world.

If your main goal is to learn something, django will not help you that much same as web2py. But when it comes to development, both will help you a lot.

If you prefer Python instead of Ruby I suggest you to start with micro frameworks like Flask or Bottle to get an idea how frameworks really do their job. And don't let that "micro" word to mislead you. You can create great applications with them.

My opinion:

Web2py is easier and more fun to use.

RoR is a lot more widely adapted, so it's better choice for your career (than Web2py or even Django). But I don't like it, because I find it bloated (like a slow and fat man) and overcomplicated.

Have you looked at Django?

Go with web2py. Its the best!!!!