I prefer the RRS MH-01 monopod head on mine. That allows me to angle the monopod to achieve maximum stability (as shown in the Nikonians monopod article) while still viewing the subject. Many prefer to use the monopod without a head. While that allows the monopod to support the weight of heavier equipment it restricts the ability to angle the monopod for added stability.
With the RRS MH-01 I can use my monopod in either mode. A ball head would accomplish the same, but IMO the three degrees of motion a ball head provides could be awkward with heavier equipment combinations.