One cannot pay directly for media attention. You get media attention if the editor deems your information 'news-worthy'. Editorial space needs to be earned.
One press release can be distributed to numerous media channels with no explicit costs involved.
Discerning customers value third-party endorsements, and view PR results as more credible than advertising.
PR cost-effectively reaches various target audience groups.
Press releases that are archived on the Internet have long shelf lives, so publicity value is extended for as long as the information is online.
With ads, media attention is paid for. You get media attention as long as your budget allows it.
Each placement of an ad requires payment, and costs differ from one media channel to another.
Discerning customers may view advertising as self-promotional and less credible, and may be more guarded when they view these messages.
Advertising is typically more expensive than PR.
Advertisements only stay up for as long as the space is paid for. Its lifespan is for as long as you can afford it.