With open source frameworks, I would choose between RoR and Django. Just stay away from anything PHP-based.
First, I would say learn the base language first, to an extent, and go with the framework built on the language you prefer. For example, I do django because I learned python, not ruby. I tried ruby when I was hired for a rails project (for frontend work, but nevermind), and it just felt too... "fluffy" for my taste.
Second, there's the issue of documentation. Django's docs are always up-to-date (keeping them so is part of the django development process) on 99% of topics you would want to look up, and are fairly easy to follow once you get the basics. The other 1% of things you would want to look up are generally completely undocumented, derivable from the source or else not possible (often possible with a third-party app, though, which are easily found). I can't speak too much for rails, but I was able to find little usable documentation in my short foray into it; perhaps I'm just too used to Django's docs.
Finally there's the development environment -- you're likely to have multiple projects on one machine, possibly with conflicting dependencies. With django/python, you get virtualenv and pip as part of the python dev process (or at least you should). These tools work very well for setting up secluded environments for separate projects. Ruby doesn't have any standardized tools for this purpose, but there is a gem called sandbox that worked well enough toward this purpose when I used it; IMO though, it seems as if it has to fight against ruby's natural behavior to get the concept to work. It looks like it hasn't seen active development in a couple years, unfortunately.
Other factors to consider include mass of third-party apps for a framework and size and attitude of the community behind a framework. In the end, though, I guess it comes down to your comfort level among the languages you know and personal preference.
And I will freely admit that I'm rooting for django.
