The best measure for any given project is the one that comes closest to meeting the criteria outlined at the beginning of this chapter: one that is most objective, most timely, most detailed (with a roll-up capability), and most predictive.
For either type of measure (percent complete or work unit), identifying the project's work units and/or activities and their completion criteria is important. In my consulting practice, this identification is typically done during a one-day workshop with the project members. When multiple contractors are involved, you may need to have separate workshops for each, especially if the development activities or work units differ (as, for example, with object-oriented versus functional decompositions). Follow-up workshops are held to work through any problems in implementing and reporting the measures.
No matter which measure you choose, be sure to keep the original baseline and then the latest plan. If comparisons are made against the latest plan only, everything looks on schedule. (I've seen projects do frequent replans so that actuals always match the "plan.") The ideal case is to use both types of measures. Together, they should give a consistent view of progress. If they do not, that encourages questions that can provide important information about the true status of the project.