rather than the framework (which largely have parity, as far as i can tell), the big decision factor in my mind is between (1) how joyful it is to code in ruby vs (2) the power of statistical/machine learning libraries in python.
if you don't have complicated statistical/machine learning needs, i'd recommend ruby on rails every time. it's so fast to get started and productive. and the language is performant enough to get you to initial success (at which point you can choose to spend money to attain greater performance).
If you do not know Ruby, it is neither easy nor fast. You have to learn a brand new language and Ruby is quite a departure from C-like languages that most people are familiar with. Then you have to learn Rails and its conventions.
It is a huge mountain of work you have ahead yourself if you are a newbie in both domains.
Sure you might be able to slap together a simple CRUD app following a tutorial but truly knowing what you are doing will take months, if not years. It is a huge investment.
if you don't have complicated statistical/machine learning needs, i'd recommend ruby on rails every time. it's so fast to get started and productive. and the language is performant enough to get you to initial success (at which point you can choose to spend money to attain greater performance).