Flip a coin and go after it. As of Ruby 1.9 speed between the two is suppose to be very close.
Ruby is the new kid on the block and Python has been around for a while.
Not that long ago I was looking to learn one or the other to enhance what I already could do with PHP. I also wanted to be able to do web development with it, and ended up choosing Ruby as I was just more readily able to find info about it in terms of web development.
However since I have started with Ruby I find that the sheer volume of documentation I was used to with PHP was lacking. I was able to overcome it though having a solid programming background before hand though. If you don't have the same kind of background I think you would really really struggle when looking at something like mechanize to scrape with.
I also ran into several gotchas as to getting Ruby 1.9 working properly under Windows, most notably getting gems (addon libraries) to compile, and just getting to integrate with MySQL took a few days until I stumbled upon the solution. Granted this is about a 1000x easier in Linux.
I'm going to stick with Ruby, I think it's a fascinating and well constructed language. With that said, I also don't think it's very beginner friendly (no matter what they try and tell you) nor do I think it's quite matured enough for the first time programmer.
I can't comment on Python personally, but the fact it's been around for so much longer I'd guess it's more stable and mature.
However I don't think that you should really chose either to get started programming. I really think you should dive head first into PHP. All you have to do is setup a AMP (Apache MySQL PHP) stack like
xampp (my personal choice). PHP is by far the easiest of languages to learn and once you've built a good base you can move onto something else. Just because you run your scripts through the browser doesn't mean it can't be used in a application type way.
This way you can also create your interfaces with html/xhtml plus other web related technologies that will grow you in other directions.
Just saying PHP is not a push over and should weigh in with heavy consideration. Don't write it off.