In all honesty, both are great. People who use either love their language and will pretty much tell you to start with the one they use. Both are free and have good tutorials online with large user communities you can go to for help. Spend a few hours with both and see which one 'feels' best to you and go with it.
I'm a Python guy so I'd recommend Python over Ruby. I just find it easier to read and that makes it easier to understand.
Ruby has a bunch of added concepts like sigils, blocks, weird operators, redefinable methods, metaprogramming, etc. These aren't terribly hard to understand if you've been programming a while, but they can be a little overwhelming for a newbie. I'd say Ruby is more suited for a Lisp or Smalltalk refugee that wants something a little more practical, not for a complete newcomer.
In terms of actual programming power, they're both good. Ruby is a little bit more flexible and provides a few more shortcuts for common operators, but anything you can do in Ruby can be done without too much trouble in Python.
Sounds like a sales wrap, but it's really how I feel about it. And it's the MIT thing to do :-)
http://swiss.csail.mit.edu/classes/6.001/abelson-sussman-lectures/
Anyway, I'd say the differences between Python and Ruby as a first language are too minor to matter. By the time differences in available libraries should matter, you should be able to decide for yourself which language to use.
Compare, for example, SQLAlchemy to ActiveRecord. Not even close. Heck, you could even compare Django's ORM or SQLObject to AR and they too tend to be more impressive.
In my mind a beginning programmer benefits as much if not more from quality libraries than a veteran programmer.
Perhaps start with Python but plan to take on another language in 2-5 years, when Ruby is more mature, or web hosting companies start supporting Scheme, or Haskell, etc.
Knowing C++ is not a requirement to, nor has much to do with, being a good programmer.
There's so much in C++ that is specific to C++ that it doesn't even begin to make sense. If you want someone to learn OO, send them to Ruby or Smalltalk. If you want them to learn about performance and deep machine magic (pointers and such), send them to C. In either case, they can learn what they need to know without getting bogged down in a really obtuse pile of bolted-on features and syntax. It's not a bad systems language, but it's not a learning language and never will be.
And the whole "you'll never be able to blah blah" crap is just a hugely stupid idea. Don't think of your learning process as preparing to do something. Pick a project and start working on it--Open Source gives you access to millions of lines of code. There is no better way to learn to program than through working on programs. Be useful as soon as you possibly can, and you'll find that people are willing and eager to help you learn, because they see that the better you become the more useful you are to their project.