If you tend to think in terms of senior vs. junior developers like me, you're doing it wrong.
Gabriel Somoza drops some knowledge on me here - the developer competency matrix.
Junior/mid/senior comes down to years of experience - period. It doesn't determine technical competency.
There can be a world of difference between two different "senior" developers - even if they work in the same technical stack.
When it comes to productivity, ability to understand requirements - these are skills that could be put into a matrix and someone has done that.