The verdict is in: Crunches ARE better than sit-ups. The reason sit-ups are harder to do is because as you raise your torso off the floor you're also stressing your lower back. From about half way up to vertical, you're using very little muscle power. So you're really only working the muscles about half the time. By only raising partially, and not letting your shoulders touch the floor after each rep, you're constantly working your abs.

Also - Quality is always better than quantity. So, instead of setting a goal of 100 crunches each day just do them until your abs are tired and burning (in that no pain no gain way). It is much more effective! Be sure you are exhaling on the way up and inhaling on the way down to the floor. Also, pull your stomach toward the floor as you exhale.