I personally read the question stem first. The main reason why is to see what I need to look out for. In a PSA question, if the stem says "what principle" I read the stimulus with the mindset, "Ok, what is a rule that this situation would apply to?" For weaken questions/identify the flaw, I read with the mindset, "This question is flawed, where's the gap in reasoning and why is it wrong?"

I think this approach helps me read with a more critical view so I know what to look for. I've tried both approaches, and when I read the stimulus first, I tend to have to go back to the stimulus multiple times after reading the answer choices. Hope this helps!