RSpec vs. Shoulda?

I am new to the unit testing scene; I have only been using unit tests for about 2 months now. When I unit test in Ruby I currently follow the TDD style and use Test::Unit::TestCase. I have also read about RSpec and how it follows the BDD methodology. I also have read about Shoulda which is a in between the two frameworks. My question is, which framework should I devote my time to RSpec or Shoulda, or should I stick with Test::Unit::TestCase? Any opinions on the subject are appreciated.