Words or the clock? If you want to be a productive writer, you need to make a commitment. In my experience, half of the working writers I know set aside a specific block of time each day for writing and half give themselves word quotas. If you have already made the choice for yourself and it's working, you know which is best for you. If not–or if you are headed into a new writing situation–here are some things to consider.
First, if you are under contract with a deadline (even if you have set your own deadline), your ultimate measure is finished copy. Your editors and publishers really don't care whether you have dedicated 100 hours or 1,000 hours to your manuscript. They care about what you put into their hands on time. So, even if that draft that led to “the call” from an agent or an editor came as the result of setting a timer each day and getting to work, you need to set some goals for completed drafts, rewrites, and final edits that add up to meeting your commitments.
This is not to say that you should throw your timer away when a contract arrives in the mail. All the rituals you have developed as a writer will help to keep you writing in changed circumstances. But you do need to add a regular way of tracking your progress.
I found that the best way for to track progress is not day-to-day. My own rhythms as a writer are not so consistent, and that daily charting sometimes can make me feel anxious. For me, hitting my goals over the course of three-day periods feels about right. For others, that daily tracking or recording progress once a week may be more appropriate.
I've also found that pages of draft per day do not equal pages of rewriting or editing. The same measures do not carry over. The biggest part of revision for me is determining the full shape of the story after it has been drafted. This involves outlining, rearranging, discovering holes, clarifying the theme, and sorting out the subplots. I don't write many words during this stage. And it always takes longer than I think it should. So I put aside a big chunk of time for this work. But, each evening, I define which piece will be completed in the following one to three days.
For all the specificity of the later stages of writing and of working under contract, my preference for drafting is a set period of time each day. When I twist the dial or push the button on the timer, it's like hearing the starting gun. And, if I have done nothing by the end of the time period, I walk away with no guilt. However, in the vast majority of cases I never hear the timer go off. By the time my designated minutes are completed, I am totally lost in the writing.
Ultimately, I think how productivity is best measured depends upon both personality and external factors. If I look at the way I have worked in the past, even back when I was a student in grammar school, I have always used a mix of time and goals. So you may have the answer on what is best for you already at hand.
What about you? Have you arrived at the perfect productivity measures for yourself? And if so, what made you choose them? If you still are not using measures, you have questions or concerns? Do you worry such measures will harm your creativity?