I've taught both, and IMHO, the Ruby is less suited for the job of an introductory language because a block (i.e. an anonymous function) is a hard concept for a beginner to grasp and a core part of idiomatic Ruby. Python's syntax simply has "less magic" and is more regular. Ruby's syntax seems to be suited for either folks who don't really want to know what's going on (just give me a DSL and leave me alone) or students who have one language under their belt and are looking for to branch out.
That said, Ruby includes a lot of interesting concepts, and while I don't believe it's an ideal first language, those concepts certainly belong in a CS program.