With effort driven scheduling, the amount of effort for each task, along with the resources, is used to determine the final schedule. It is the effort that drives all the planning activities. Changes in effort estimate require consideration for changes in the scheduling. This is perhaps the most accurate way of scheduling as it looks at each task and project independently. If any tasks or work is combined, the tasks are within the project.
Activity based scheduling is popular in "job shop" environments. It looks across projects and for opportunities to combine related tasks for efficiency. The issue here is that the scheduling is far more complex and there is a higher risk one or more of the projects may not be delivered on the original schedule. If timely delivery is not critical, then this might represent an opportunity for cost savings.