Both tools are valuable, even for the same subject area. For example, in mathematics courses, an asynchronous delivery format is valuable in allowing a student (at his or her own pace) to review (and review again) lectures. Then, as the student progresses to the next stage of working through problem sets, the asynchronous forum serves as a reference resource while the synchronous forum (perhaps a Skype session or e-whiteboard session) allows for real-time interaction with other students or the instructor. The two delivery systems are complementary, so the question is not "which" delivery mechanism, but "when" to utilize each during the course.
