I think we should all agree on a definition of developer first. Not just in the sense of "HTML isn't a language", but picking a specific domain in which to compare developers.

I don't think the concept of productivity can span multiple fields. Does a C programmer have the same productivity as a Ruby programmer? On the other hand, maybe some part of a Ruby project needs optimizing, so a routine is written in C. Maybe there can be a definition of productivity that crosses languages... but we don't have one yet.