Your choice comes down partly to a tradeoff between response rate and referral power. Analysts receive plenty of networking messages, but they are generally more accessible than senior bankers. They also remember recruiting more clearly and can explain interviews, staffing, group culture, and the day-to-day job in practical terms.
Managing directors, or MDs, lead client relationships and help generate business. Their seniority can give a recommendation significant weight, but they have demanding schedules and may be selective about student outreach. An MD referral can be powerful; an unanswered email is not.