There is only one web-based language and that is Javascript. With node.js and Rhino, it can now also be used on the server side.
So if you really want to learn a web-based language Javascript is the one. If you decide to go for Python or Ruby (good choices) or even PHP or PERL (bad choices) you still can't escape from learning and using Javascript.
Personally, I think that you should pick Python or Ruby, whichever seems to suit you, since both are solid languages that are widely used to build web apps and lots of other stuff. PHP and PERL are yesterday's languages and are fading away slowly but surely.
So if you really want to learn a web-based language Javascript is the one. If you decide to go for Python or Ruby (good choices) or even PHP or PERL (bad choices) you still can't escape from learning and using Javascript.
Personally, I think that you should pick Python or Ruby, whichever seems to suit you, since both are solid languages that are widely used to build web apps and lots of other stuff. PHP and PERL are yesterday's languages and are fading away slowly but surely.