Changing the upper only is cheaper and allows you to also change the noisey stock coupler to a quieter solid one.
Changing the lower requires you also change the harmonic balancer so that's the added cost.
The upper does require removing the blower front snout as the pulley is pressed on.
Beyond that, they accomplish the same goal of spinning the blower faster for more boost and some times are done together because going really large on the lower requires moving things around to make room (other pulleys, hydraulic lines) so a combination with a medium lower accomplishes the same boost as a large lower alone.
Hope that helps!
