As most already said, there is no need to compare things that serves completely different purpose. I see review campaign more like a way to buy some positive comments about service, like paid shilling. But I'm not really sure how much in helps to attract users.
I think you're wrong about signature campaigns. It's not about to attract campaign participants to use service. It doesn't makes much sense and it would be too expensive. It's more about giving visibility to brand, like billboards in streets or ad banners on websites.
I'm not sure where you get these numbers from. Is there any questionnaire or research that talks about this? From my experience, I tend to use the service I advertise on my signature occasionally to make sure it still works properly while I tend to forget a project that I reviewed unless I really need their service. If your number is legit, wouldn't you say that the signature campaign is better at customer acquisition then? 5% of participants don't use the project means 95% continue to use them, no (assuming the participant number is the same)?
Probably it's just some random numbers from his head as I doubt that someone has research about these things.
I agree with you about using service that I advertise in signature because I want to know what I'm advertising.