I think you might be confused between Ruby and Rails.

Ruby's current version is 1.8.6, while 1.9 is nearing an official release and 2.0 is still very experimental at this point.

Rails' current version is 2.0.2, but the last release on the 1.x branch was 1.2.6.

I'm going to assume that you meant Rails 1.2.x versus Rails 2.x. Yes, there are a lot of advantages. Mainly though, there aren't really any disadvantages- there's no reason not to use Rails 2.0.