Ok, admittedly, that title is quite cryptic, but couldn't imagine any better short explanation.
What I mean is the following: The (expected) performance / qualification of employee A is MUCH higher than that of employee B for a given task X. The (expected) performance / qualification of employee A is also higher than that of employee B for task Y, but the difference is not that big.
In short, the general assumption (at least one that I have - may be discussed in another question), that 'everyone has his strengths and weaknesses' does not apply here, to the extend that A is considered to be more qualified / efficient in task completion than B.
Based on which criteria do you decide whom you assign to task X, and whom to Y? Priority and importance of the according tasks? Even though this may sound a little to rational, do you think the concept of 'comparative advantage' may be applied here (disregarding the fact that true qualification is hard to be quantified)?
Does your decision change if task Y is more important and of higher priority than task X?