I'd go for PEEP. Provided the stem has Fio2 within an acceptable range, lowering tidal volume would be inappropriate because the patient is mostly likely has hypoxemia, and we don't want alveoli collapsing and collecting secretions to further aggravate the A-a gradient. And usually, normal weight-adjusted tidal volumes should be fine in ARDS patients. I think the answer will depend on the scenario being given, whether the patient looks fine with good saturation and you're more concerned about the barotrauma risk than oxygenation.