PR vs Advertising

How much of your budget should go to PR versus advertising? For most companies PR is more effective than advertising.

Advertising does very little to build a brand, develop the message, and gain trust for new or not well known brands and products; it does a much better job maintaining brand awareness, which is why big brands spend so much on ads. Notice, however, that even though Microsoft outspends Google by better than 60:1 they only have about 10% of the search engine market compared to Google's 50%. PR, including viral and word-of-mouth messages, does more to build the brand in part because a friend's good experience does more to convince us than an ad.

There's another difference between the two: trust. Advertising has lost consumer trust. This was recently demonstrated when San Francisco installed new solar-powered transit shelters:
Preston at Jetson Green calls it "an interesting collaboration of five different organizations: San Francisco Municipal Transportation Agency(SFMTA), Clear Channel Outdoor, Inc., Lundberg Design,3Form, and Konarka." They are lovely things with Konarka flexible photovoltaics integrated into the 3Form recycled plastic roofs.
So what's the problem? The shelters are paid for by Clear Channel advertising. Lloyd Alter, in blogging about the shelters, opposes them for this reason, citing their contribution to ad creep:
When you give away the land for a shelter here, a garbage bin there, pretty soon your City is covered in ad creep, ads everywhere, more like Blade Runner every day. Private companies don't "give" us roads and sidewalks; we pay for them. Why isn't decent shelter while waiting for public transit as much a public responsibility?
We don't know yet how the majority of San Francisco citizens will respond to the shelters, but those agreeing with Alter have an automatic, strong, and negative disposition towards any product or brand featured in one of these ads. Even for products they usually would like. Meanwhile 3Form, and Konarka are gaining free publicity and look like heroes for their commitment to sustainability and contribution to the city.

Negative responses to paid advertising versus positive responses to free publicity; which of those works better for your company?