  We study the ordering of two-qubit states with respect to the degree of
bipartite entanglement using the Wootters concurrence -- a measure of the
entanglement of formation, and the negativity -- a measure of the entanglement
cost under the positive-partial-transpose-preserving operations. For two-qubit
pure states, the negativity is the same as the concurrence. However, we
demonstrate analytically on simple examples of various mixtures of Bell and
separable states that the entanglement measures can impose different orderings
on the states. We show which states, in general, give the maximally different
predictions, (i) when one of the states has the concurrence greater but the
negativity smaller than those for the other state, and (ii) when the states are
entangled to the same degree according to one of the measures, but differently
according to the other.
