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.
Whereas, we can say that upto 5% of signature campaign participants doesn't use the project they promote.
This is not an effort to undermine signature campaign because it may have a better influence than review campaign outside the forum.
I am not so certain about this, I will like to hear from project owners and campaign managers like Royse777 who often launch review campaigns.
This is like you asking the obvious, it's practical in the number of reviews you see on the forum compared to the number of campaigns you see. If campaigns don't pay more, you will definitely be seeing more reviews as the companies are not in for wasting money and time, they are in for a serious business, real marketing and more conversion possibilities.
A reviewer is just one person and it doesn't matter if they use the product or not, and their review might only be written once in a single section of the forum which limits the number of viewers. But a campaigner might be one yet has multiple presences at many sections of the forum, sharing the project link and thereby propagating the message as much as possible. This could cause a single campaigner to attract over 100 accounts opening in just a few months. I hope you see what is naturally productive among the two.