The difference between outright paying and giving discount is as follows:
Paying for reviews would indicate to the customer that there is something wrong with your services, for which you have to pay for others to come to them. May be it was his good day that he had a good meal.
However, asking them for review and giving them discount is an invitation for two things:
1. You want to know what was wrong or right in their experience at your place. This will give them an indication that you are looking to work on your services and improve it for the benefit of the consumers.
2. By giving discounts, you are trying to invite them back at your restaurant. A customer will feel that with feedback given, they might get a better experience than what they had last time, in spite of paying less for the same.
Hope this was of some help