Product-based rewards translate directly into higher product engagement. This combined with word-of-mouth marketing, can dramatically boost your credibility and nudge more than a few hesitant clients to use your services.
A free month of subscription is more of an incentive than a subscription discount and might get the referees to start using the service free of charge to get hooked on your product. This is especially important if you do not offer a free trial. Giving your product subscription is relatively cheap and ensures that your customers will stay loyal to your subscription-based service or product.