The verdict
If you bill through Stripe and want to be live fast with little engineering, pick Rewardful. The vendor says set-up takes under 15 minutes, and it does a narrow job well.
If you bill through Paddle, Recurly, Chargebee or Braintree, or you want lifetime recurring and tiered commissions with clawback on cancellation, pick FirstPromoter.
On Stripe alone it is close, so I do not name a winner. Look at the revenue caps on each plan against your forecast, because a programme that works moves you up a tier in both. Neither recruits affiliates for you.