The "quickly" argument is a strawman, if you plan on having a working relationship that lasts longer than 3 months or so. If you just need someone to upgrade your current Rails 2.3.8 project to 3.0.6 and then move along, by all means get someone with Rails experience.
But if you intend to employ that person longer, the specific experience is of diminishing importance.
That's why I usually try to describe myself as a bard of programming. Pick a language. Pick a framework. I know enough to get started, and I know where to find answers to my questions.
But if you intend to employ that person longer, the specific experience is of diminishing importance.