TDD vs. BDD...How and where to start?

About a month ago I switched back to Rails 3 from doing almost entirely PHP (codeigniter) web app coding. I'm loving the speed at which Rails lets me bring things to life, and I've been spending time trying to absorb "good" Ruby practices by reading books like "Metaprogramming Ruby" to get the core fundamentals down.

In order to take my Rails coding to the next level, it appears I need to start incorporating some testing into my apps to make sure things stay resilient as I expand and add features. I understand the core ideas of why test frameworks are good, but I really don't have any idea of which one is best and where to start.

It seems like Test::Unit and RSpec are the two big players for the Ruby world...can anyone share advice about which one to start with and any good resources I could use to get moving in the right direction?