We usually use leaderboards for limited-time referral competitions where, besides the regular incentive, we set one or multiple grand prizes (e.g. get $10 for each friend you refer + top referrer gets a $100 prize).
Tiered rewards campaigns are used to motivate advocates to share more on a regular basis (e.g. refer one friend - get $10, three friends - $25, five friends - $50, so the advocate gets $85 instead of $50 for five referrals).
The leaderboard is better for a shorter-term promo with grand prize(s) while tiered rewards campaigns are more effective for longer-term promos.
Of course, it also depends on incentives and your preference, so feel free to try both alongside other events on your marketing calendar!
You can find some examples in this deck.