Personally, I feel that review campaign brings more results to the project owners because the participants of the review campaign will surely use the project and there is the possibility than more than 30% of the campaign participants will continue to use the project if it is a good project.
The target of the campaign is not the participants exposure to the product launching their campaign but rather to give project a better exposure to the general audience. Review campaign main goal is to get feedback to the service they are offering. Review campaign and Signature campaign have different goal.
Signature is for exposure while review campaign is to gather feedback. I doubt your 30% figure is true or else project will choose review campaign as fix advertisement since 30% conversion is guaranteed. This is 2 different campaigns with different goal, you are the only one who assume review campaign as a better campaign for exposure compared to signature campaign that literally spread the brand all over the forum. Review campaign exposure is limited to a single thread.