Choosing Between a Diary Study and Interviews for a Habit-Forming Feature

You're researching whether a new habit-tracking app feature actually helps people stick to a daily routine over several weeks, or whether people abandon it after the novelty wears off. You have budget for either: (a) one round of 60-minute interviews with 10 current users, or (b) a 3-week diary study with 10 participants logging short daily entries about their use of the feature.

1. Which method fits this question better, and why does the other option specifically fail to capture what you need?
2. What's the single biggest operational risk of running the diary study, and how would you mitigate it?
3. Would you run this generatively or evaluatively? Justify your answer.