I am a little confused by the sheer number of testing frameworks available for Ruby/ROR.
I have recently watched the Cucumber Railscasts and found them very interesting.
From my experience, Cucumber and Rspec have different appeal. Rspec appeals to me from a developer perspective because its easy to write and provides very quick feedback when something breaks. Cucumber does not appeal to me as a developer because it does not run as quickly as Rspec. However, Cucumber does appeal to me as a business stakeholder since it provides full coverage of entire features.
Do yourself a favor and keep writing unit tests.