Whatever results in the less convoluted and easier to understand syntax.
Usually people find it easier to understand active voice. Even research articles are today usually written in active voice and avoid confusing self-reference-avoidance (do: "We conducted a study...", don't: "A study was conducted ..." [by whom?]). Machines are operated by persons, they are not "being operated" by a mysterious force.
The APA Manual – which, admittedly, aims at scientific manuscripts, but is often more widely applicable – has this to say about passive voice:
... the passive voice suggests individuals are acted on instead of being actors ("the students completed the survey" is preferable to "the students were given the survey or "the survey was administered to the students"). (p. 73)
Dangling modifiers have no referent in the sentence. Many of these result from the use of the passive voice. By writing in the active voice, you can avoid many dangling modifiers.
Correct:
Using this procedure, I tested the participants. [I, not the participants, used the procedure.] (p. 81)Incorrect:
The participants were tested using this procedure.
But the passive voice is a valid mode and has its uses, too, of course:
The passive voice is acceptable in expository writing and when you want to focus on the object or recipient of the action rather than on the actor. For example, "The speakers were attached to either side of the chair" emphasizes the placement of speakers, not who placed them—the more appropriate focus in the Method section. "The President was shot" emphasizes the importance of the person shot. (p. 77)
