I'd do both. You definitely need research on situational factors so that you can use it as a comparison, and it offers more ways to show critical analysis i.e. by pointing out the strengths and flaws in environmentally focused studies as well as personality focused. The question asks you to discuss in relation to Milgram's work, so I'd link that in all the way through. Personally, I'd structure this essay by starting out with explaining situational research and Milgram's study and explaining the issues with it, which would lead into why personality factors should also be considered etc...hope that helps!