Book publicity is the effect of being chosen, whether it's by an agent, editor, etc. This automatically gives your book credibility. Having a third party verify your writing skills is usually more effective than an ad campaign, because in this way you are getting people to talk about your book rather than you telling them.
I remember seeing an article that said, statistically, 84% of people talk about a book they read. I wouldn't disagree with this number. That being said, if you get one person interested in reading your book, odds are you'll get at least two people who know about it. Generating authentic interest is crucial to creating public interest. This is why so many emphasize the critical importance of quality content. If you're writing isn't up to par, then you're going to have a more difficult time selling your book.
If you choose to embark on a publicist, be sure to research them and ask them questions. They don't come cheap, but you can learn things along the way. Some are better than others, in the sense of experience and contacts. Be sure you're making the right decision, because a PR campaign is something that is very serious, and you will always be the biggest advocate for your work.
If you choose to embark on an ad campaign, be sure that you're targeting an audience as opposed to simply putting an ad up for all to see in a random location. You'll see more interest in your work.