We study a multiserver system where servers are heterogeneous in terms of both their speeds and their usage (energy) costs. We show that the optimal control policy is threshold type, and can be implemented in a decentralized manner through an individually optimal policy. When there are no arrivals, we also derive explicit expressions for the thresholds. We consider both the case with reassignment, where jobs can be moved from one server to another at any time, and the case where reassignment is not possible. When reassignment is possible, determining which server is preferred (i.e., the one that is used even if there are very few jobs waiting) is surprisingly difficult. With arrivals, for the reassignment and no-reassignment cases, determining server preference is also difficult, and depends on the arrival rate.