This is a false dichotomy: my suggestion and yours are orthogonal (except perhaps for resource allocation consideration).
You are right of course that proper debugging is desirable (MCL also had that 20 years ago), perhaps more so than what I am suggesting, but I'm going for the low-hanging fruits here. Moreover, adding Ruby debugging to Xcode would require Apple willingness
to evolve Xcode which, given the glacial pace of Xcode 4 bug fixing and almost total deafness to developer requests, is far too much to ask. What I describe on the other hand, require no such cooperation. I probably even could implement a rough bare bone version.
Jean-Denis