IMO hitting up the calves with lower AND higher reps is optimal. This can be done in several different ways, going heavy in one workout and ligher in the next, or doing both in the same workout. Heavy range is anywhere from 6-10 reps and lighter anywhere from 15-30 reps.
The soleus should be primarily targeted with higher reps however (seated calf raises), because of the dominant slow-twitch makeup of the muscle. So it should be trained with reps as low as 12 and as high as 30.
Sample workout:
Standing Calf Raises- 3 x 8-12
Seated Calf- 2 x 20