Much research in the last few years has been devoted to development of efficient structured peer-to-peer (P2P) overlay networks, which offer distributed hash table (DHT) functionality. Most of these systems have been devised as flat, non-hierarchical structures, in contrast to the most scalable distributed systems of the past. To cope with this, a significant number of hierarchical DHT designs have been proposed in the literature. Unfortunately, no design is "universally" better. Actually, what is lacking is an analytic framework to identify the good hierarchical design for a given workload. In this paper, we provide such a framework, and we use it to compare the two main hierarchical DHT designs: The homogenous design, in which all nodes act equal roles, against the superpeer design, in which a small subset of peers (i.e., the most powerful and stable), behave as proxies, interconnecting clusters with highly dynamic membership. Our analysis reveals that, on the contrary to what was initially expected, the costs incurred by hierarchical superpeer design are not necessarily minimized.