If abab lets you start being productive sooner, then abab is more productive. It may or may not be more efficient than B-before-A -- I doubt we can conclude on

If abab lets you start being productive sooner, then abab is more productive. It may or may not be more efficient than B-before-A -- I doubt we can conclude one way or the other in the general case, so I don't see the point in discussing the two approaches' efficiency.