Best practice for computing week-over-week change in MAU metric
Both of these options have value if interpreted properly. You are trying to answer  the question (I assume): 'are we doing better this week than last week?'

I don't know what industry you're in or how variable (or volatile) your metric is, but I'd advise caution in interpreting changes week to week. I might suggest that alongside your weekly (or whatever) metrics, you include some moving average. That way, you capture the overall trend while tempering your reaction to occasional ups and downs.