In most contexts, choose the active voice because it is clearer, more emphatic, and less wordy than the passive voice. Active constructions clarify who is performing the action.
passive Election results are often determined by a small number of voters.
active A small number of voters often determine election results.
In some contexts, however, the passive voice is preferable precisely because it deflects attention from the actor.
Smoking is prohibited. [Contrast, for instance, "The management prohibits smoking." (active voice)]
A shift between active and passive voice in a sentence can be awkward.