I have two employees, Alan and Beth. If I assign a task to be completed within 4 weeks (which is enough plus some slack to spare), both will finish on time with the same quality of work. The big difference is their approach.

From the beginning, Alan starts selling up the amount of work required to try to lower my expectations. He tries to make out his work as some sort of Herculean task. I have worked with Alan long enough to know his capabilities, and know that the schedule gives plenty of time to finish (and then some). While I know there's slack, Alan works hard to give the appearance of working hard all the time.

Beth doesn't push back and just gets the work done on time. When there's slack to spare, Beth may browse Reddit or otherwise putz around on the net (or leave early, or take long lunches). The work all gets done, but the slack is glaringly apparent to both me (and everyone else).

![Visual Representation of Alan and Beth's Effort](https://i.stack.imgur.com/GjQdT.png)

At the end of the day, the results are the same. Both are using the spare time in different ways, but both get the job done to an acceptable standard. My personal bias makes me prefer Beth's approach, only because I feel like it's more honest, but I am worried that I am just more approving of qualities I share rather than taking an objective approach.

What reasoning, if any, would justify giving different performance reviews to employees who provide the same quality of work, are the same caliber of employee, whose only difference is their approach?

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Clarifications:
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> 
> If you are judging staff be their output and assigning tasks to 2 employees that need to be done within a reasonable period of time, and both employees have completed the tasks then for all intents and purposes *they are the same.*
> 
> 
> 

(from [Lego Stormtrooper's answer](https://workplace.stackexchange.com/a/13482/7945))

I am not managing contractors (paid to get a specific task done in X amount of time for Y budget), nor am I managing factory workers (who have a clearly objective quota of widgets to finish in a week). Part of my job as a manager is to treat full-time employees like Alan and Beth as assets for the company, and to help develop their value not so they can just get this project done at an acceptable level, but so their value as assets of the company will grow long-term.

If I judge them solely on output, by those standards I am not doing my job. Let's say my company were to ask me to layoff one of those employees. I would have to determine which has more value to the company -- not which one completed their tasks more effectively last week (and if the same, flip a coin or draw straws).

To keep employees happy I need to be fair (or at the very least perceived as fair by my employees). If it was a layoff (where things are rarely fair) I would value Beth over Alan in a heartbeat (because there is more obvious slack to pick up additional duties where shorthanded), and I don't think that would have a negative perception (beyond the fact that it's a layoff). But in the case of a performance review, I am wondering if there is a similar way to justify it while keeping the image of being 'fair'.