Hey! Someone actually sent me a question, per my request below...
"I run a start-up. How do I know whether to advertise or use PR?"
First, the biased answer: "PR is always the right answer."
Now the nuanced answer: "PR is almost always the right answer."
Here's the essence of it, Mr. Start-up: With advertising, you can completely control the message; it takes relatively little of your time (assuming you are not a control freak in the ad design phase); you can place this tightly-controlled message wherever ya want. That's great - but the credibility factor hovers somewhere around "zilch."
Now, despite what you may have heard about the General Public's disdain of traditional media, outlets ranging from PEOPLE to eWEEK still wield enormous influence over purchasing factors. And while many pundits claim that people don't read, that they're barraged by too much info, the reality is that most folks on the Internet are reading something, goshdarnit. ...The difference from Ye Old Worlde is that they can now read whatever they want to read - there are no printing press limits and dang few editorial filters - and, they can react to it in an organic, impactful way.
And this, friend, is the beauty of PR: PR is about credibility, and PR can have an effect on the aura of credibility in virtually any forum, whether a blog or a print magazine. PR can be as crude or as polished as it needs to be, and, since it is usually all accomplished with words and 1:1 contact (which leads to 1:many distribution), it can all happen on-a-dime.
It is true, though, that as in the world of High Finance, there's a risk:reward ratio in PR. With PR, you can have much higher levels of credibility, but at greater risk (you can't control the message, and woe to you if you try too much to do so!), and, at a far greater expense of time (advertising is primarily event-driven. PR is non-stop, 24/7, forever).
By the way, did I mention that PR is a lot cheaper than Advertising? No fancy models or photographers or media placement firms in PR's mix. Since you likely have a limited budget, as a start-up, this is a consideration.
As a start-up, your first goal is credibility. PR can give you that, for a lot less. It's riskier and more time-intensive, but it's the way to go.
Having said all THAT, I am not one of those PR types who hate the Ad guys. Ads are cool. Ads are a great way to bolster an existing and credible corporate brand. "I'd like to buy the world a Coke"? - BK's "Subservient Chicken" campaign? - C'mon, that's great stuff. When you are a name-brand company, Mr. Start-up, you should definitely augment PR with advertising campaigns.
Want more? Here's a great book on this subject. It's called "The Fall of Advertising & The Rise of PR" so yea, it is biased - but in a smart way, and it's a book written by a noted brand expert, Al Ries. You want a book that skews in favor of advertising? Sorry, wrong blog. ;)